The PCs are testing the rumor—so make a choice!
Your campaign is a custom one, so while purify food and drink maybe should apply only to food and drink that's consumable by a creature of the race casting the spell or by the god that grants the spell or something, that doesn't mean the campaign's pixies aren't actually food for everybody.
In other words, the PCs have heard rumors of a creature that gains power via snacking on pixies, so when the PCs cast purify food and drink on a pixie corpse and they or their goat friend is about to dig in, you've to decide if those rumors of greatness through faerie feasting are true. (Fun Fact: PCs—and players!—will likely be disappointed if such rumors are untrue.)
If the rumors aren't true, the spell simply fails. No harm, no foul, no pixie picnic, no übergoat. All done.
If the rumors are true, then you've to decide how true. Can anyone partake of this pixie power-up? Need the pixie be alive? (Ew.) Some degree of fresh? (A day sounds good.) Must the entire pixie be consumed? ("Pixie wings taste like fruit roll-ups!") Is that healthy? (Likely not.) Must pixie be prepared a certain way? ("It must be served... on a stick!") Do a certain number need to be consumed to gain ultimate pixie power? And so on. Then the spell works, but, maybe, after consuming the pixie, the PCs are left wondering why their goat has a bellyache instead of awesome pixie powers.
This is your campaign, and—to challenge the frame a bit—this is less about how the spell purify food and drink works and more about the effects of pixie-eating in your campaign. That is, determine event's outcome and steps required to reach it first, and the question about the spell likely answers itself.
The spell does not say, so it is up to your DM
You create 45 pounds of food and 30 gallons of water...the food is bland but nourishing...
There is no indication in the spell what the food consists of. As such, you must talk with your DM and decide if it makes sense for the spell to allow for the creation of bland but nourishing meat.
As Jeremy Crawford has said:
A spell's text details the spell's effects—the only thing the spell does. Any additional effects are up to the DM.
In my opinion there is nothing game-breaking about allowing a food-creating spell to create some sort of meat. It is, after all, the purpose of the spell to create things to eat. In other words, I see no reason that would justify restricting player freedom and choice in this case.
"Nourishing" food could even make meat vs not-meat irrelevant
In fact, I think it is even reasonable to read nourishing ("containing substances necessary for growth, health, and good condition") as indicating that it doesn't even matter. If the food is nourishing for anybody that eats it, then even a carnivore would be able to subsist on it no matter what form it takes. (If a carnivore can eat something and find it nourishing isn't it essentially meat?) As a DM, this means that I would see nothing wrong with allowing the player to theme the food/water however they like.
However your DM may disagree. Talk with them and work it out.
Best Answer
'Rules as written' it's pretty tenuous, 'rules as intended' it's definitely out. Allowing this could easily break your game.
Before I get into the details, consider the general principle that there are no secret rules in D&D 5e. So, always be suspicious of a reading of any feature that seems to allow additional benefits for only a small subset of users, especially if those additional benefits are not explicitly stated anywhere. Anyway...
1. Purify Food and Drink
Rules as written, whether a humanoid eating character could use this spell to heal other humanoids of poisons and diseases really depends on how you choose to define 'food and drink'. These are not game defined terms, so it's up to DM to make their own call or ruling. Can living breathing humanoids be considered food? That may come down to whether you'd call a cow food, or whether it would have to be a steak first.
Rules as intended, this should definitely be disallowed - it's a huge exploit. To illustrate this just compare the effects with Lesser Restoration. Lesser Restoration is a 2nd level spell that lets you cure just one creature of just one condition. Purify Food and Drink is a first level spell, which can be cast as a ritual, therefore not expending a spell slot. Ruling that it could be used this way would allow it to cure every poison or disease affecting every creature in a five foot radius.
2. Create Food and Water
Rules as written, it's just about arguable this spell could be used by a humanoid eating character to create an adult halfling. If you have already ruled that living humanoids can be considered 'food', which I would not recommend, then, by weight creating a living adult halfling is possible (they weigh about 40 pounds).
Would a halfling be bland and nourishing to eat? Nourishing, maybe but bland seems unlikely - however as far as I'm aware there's no published material that discusses how the playable races taste. If your DM has already allowed you to consider a living halfling 'food' it seems unlikely that this will prove a sticking point, with a lack of actual evidence to the contrary available.
Rules as intended, however, once again this is a massive exploit. The spell text states that the 'food' you create 'spoils if uneaten after 24 hours' - that doesn't seem like an accurate description of a happily functioning, living and breathing humanoid to me. The spell text therefore probably isn't intended to describe living creatures.
Once again, comparison to other spells reveals how overpowered this is. There are lots of other spells that allow conjuring allies (for example) and they generally are fourth level or higher spells, and once summoned the allies stick around for about an hour. If allowed, Create Food and Water, would undercut all of those spells. It would become a third level spell that allowed the creation of a halfling ally that could last a day until they started rotting (or years if you then cast Purify Food and Water on them!). Either way, by allowing all of this you'd have moved firmly into fairly ridiculous homebrew territory.
All other spells that are capable of summoning companions provide guidelines for the statblocks that should be used for those companion. Here you'd need to make them up - which should be another indication that it's not an intended use of this spell.
3. It's not about the Ogre
A final caveat - as I see it this question isn't really just about an ogre. The ogre is (ironically) the thin end of the wedge.
As demonstrated in this Reddit thread an NPC Ogre wielding these spells could be a hilarious addition to your campaign. However, what's good for the goose is good for the gander and if an NPC can do it, soon your players may want to do it too.
Ogre's aren't (at time of writing) a playable race but there are playable monstrous races which could be argued to eat other humanoids. Taking it further, in an evil campaign, any PC could declare themselves a cannibal. Or, in a good campaign, they could take the Haunted One background and say that as a child their PC got lost in the wilderness and ate their friend, after they'd died, in order to survive. They've never told anyone but the memory still tortures them and they can't shake the idea that ultimately everyone around them is made of food.
Basically, if you let an Ogre NPC do this in your campaign, it'll be hard to justify disallowing other characters - including PCs - from doing it too.