One could certainly say that Bottom is a fool in A Midsummer Night's Dream, especially since he seems relatively oblivious to the reality of his situation throughout the play. For example, Bottom is too oblivious and dim-witted to understand that, when Titania and her fairies lavish him with attention, the joke's really on him. Puck purposefully bewitches Bottom and Titania in order to play a prank on the fairy queen (and provide some hilarious entertainment for himself in the process). Bottom, however, considers the fairy queen's attentions to be genuine, and so he is foolishly unaware that both the audience in the play (Puck) and the audience watching the play are laughing at him the whole time.
However, it would probably be unfair to say that Bottom is only a fool and nothing else. For instance, he's also an energetic and excitable fellow who's passionate about the rude mechanicals' play. While his acting isn't very good, and while his excitement about such a rudimentary performance certainly makes him seem foolish in many ways, his genuine enthusiasm for his part should make us occasionally regard him with a little more charity.