Is it bad to eat too much yogurt?

If it's too much, then that pretty much automatically means that it's bad.

But if you're really asking about a lot of youghurt, then the answer is; so-so.

Most youghurts would have quite a lot of calories in them, both from fat and from the sugary fruits often used for seasoning.
So if you eat a lot of them you might get more calories than you need, which will eventually make you overweight.

Also, it'll be real hard to get all your nutrients from youghurt alone, so you might end up with deficiencies if that's all you eat.