In general, riders and mounts can be targeted like any other creature

The rules for Mounted Combat don't specify how targeting works for the Attack action (or any other action) because targeting riders and mounts is no different that targeting any other creature, with the exception of opportunity attacks.

Whether you can target either, neither, just the rider, or just the mount has nothing to do with their status as rider and mount, rather it depends on many other circumstantial factors, the most common of which are by far range and total cover.

From Making an Attack:

Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

From Total Cover:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

For example:

• If your attack has a 5' range and you are a medium creature standing at the feet of a T-Rex, then you can target the T-Rex, but you probably¹ can't target its rider.
• If you can see a rider peeking over a wall, but its mount has total cover, then you can target the rider with an attack, but not the mount.

Opportunity attacks

The rules for Opportunity Attacks state that:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

And that:

You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

The rules for Controlling a Mount also state that:

if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.

In short, a rider never provokes your opportunity attack when it moves using its mount's movement. But if a mount provokes your opportunity attack, then you can choose to target the rider instead of the mount, despite the fact that the rider did not provoke said opportunity attack.

However, this does not mean you can target a rider regardless of all other factors such as range or total cover. For example, opportunity attacks occur retroactively just before the target leaves your reach, but if the rider was never within your reach to begin with, then you can't possibly attack the rider.

1. The height of creatures and the location of a rider atop a mount are often not well defined so there is room for DM adjudication.

CC takes the hit.

The horse is the one provoking the attack of opportunity, no matter who the attack eventually targets.
After the attack of opportunity has been provoked, the troll is simply making a melee attack against a target - and the Mobile feat does not provide you immunity to being targeted by melee attacks.

Some context about opportunity attacks, mounted combat, and the mobile feat:
When riding a mount, you are moved by someone without using your movement, action, or reaction, and as such you do not provoke an opportunity attack.

You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. (Chapter 9, Making an Attack - PHB)

Mounted combat also specifies that an attacker can choose to target you with an opportunity attack even when you don't provoke one, if your mount provokes an opportunity attack.

if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount. (Chapter 9, Mounted Combat - PHB)

So in actuality, the Mobile Feat provides no real benefits while mounted:
You are using your mount's speed to move, so the speed increase does not matter;
Your mount would be the one taking the dash action, not you;
You already do not provoke opportunity attacks when mounted.

And aside from that, the discerning rider might want to have his mount Disengage as its action, avoiding this situation entirely.