Yeh Sweet Tooth's level 2 is seriously garbage. I have never found a reason to use it. I know that it can get three people if they're bunched up right next to you, but that's not even a risk worth taking. Sometimes it's harder to even just get ONE kill with it compared to his level 1. It's ridiculous, I don't know what they were thinking, I guess they just tried to give him a garbage level 2 to balance out his level 1 and 3 supers, but it seems kind of dumb.
His level 1 is really nice though, and his level 3 has the potential to honestly be the strongest level 3 in the game (and I feel like the developers believed this too, hence his ass level 2). The mini-gun is monstrous and and the super lasts long enough to get 7 kills (or possibly even more).
The only problem is like others have said, it's easy to avoid it by getting under his feet. Usually if I've got people under my feet, I just continue stomping and eventually they get hit by it. It is pretty easy to dodge, but usually you'll still catch them eventually. It just uses up so much time trying. And the rockets are near useless. They need a larger spread.