As far as I know, it should count towards the normal campaign. My friends and I have played the mutations every week so far. The first week, we were all able to get the achievement for crossing the bridge in under 3 minutes in The Parish because there wasn't a full number of common infected. We also all received credit for chainsaw kills during the chainsaw event, so I presume that you should be able to get full credit for Confederacy of Crunches.
This problem has different solutions depending on how you want to play.
If you're just looking for the achievement, one solution to your problem is to do whatever missions you want, choosing the side of good, until you get relatively close to your level point. Then go into a town and steal (for minor karmic reduction) or lay waste to entire towns (for major karmic reduction). You can quickly lower your karma this way. Then, once you're neutral, go kill some random creatures until you level.
(If you're looking to steal, a good place to do it is at Dukov's place… he's got stuff everywhere.)
The "nice" thing about this system is that if you stay on the path of "good" the whole time, you can do this at each achievement point: be bad until you get to the right karmic level, level up, get your achievement, and then restore a save to get back to your good self. You can quickly get "good", "neutral", and "evil" achievements for all levels on one playthrough this way.
(I did this, because I don't really enjoy long-term playing as an evil character… but I do like my achievements!)
You could also do this as an evil or neutral character, but it's much easier to quickly drop from "good" down to your new goal, than to work your way up.
If you find that you're close to leveling, and you're at the right karmic level, and don't want to take a chance on changing your Karma by doing a story-type adventure, you can always go wandering looking for random bad people to take out. Going near the Washington Mall can get you some XP fairly quickly, and you can go inside the capitol building and clear out large areas without picking up any Karma, positive or negative.
Best Answer
I'm not sure if consuming corpses directly counts. Even if it doesn't, once you've consumed 25, you'll gain the Dine and Dash challenge perk, which allows you to harvest Human Remains, which can be used from your inventory, and are most definitely food.
Ghastly Scavenger is unaffected by Dine and Dash, so if eating corpses directly doesn't count, then I suspect the same would apply for super mutants et al. The easiest way to test this by the way, is to take the perk, and check the challenge counter viewable on your Pip-Boy before eating a corpse.