In a dictionary, this tells you that the word/phrase has a negative connotation. If you use 'suck up' to describe somebody's behaviour, for example, I will assume that you do not think this behaviour is good.

Disapproval is to not approve of something, or to dislike something you would so disapproval. For example, if a kid was to do drugs and you don't like kids doing drugs you would show disapproval. Suck up to could be negative or positive depending on how you look at it. To suck up to someone means you are trying to do anything you can to get on their good side or if you want something from them. For example, would suck up to my teacher to try to get a better grade.