He only fears he is - this was ony in his head
In reality (the show's reality) we saw Angel dancing as well as he can be expected in The Prom. But at Cordelia's party he panics and imagins the worse case scenario which we saw.
DB said once in an interview that the humor in Angel comes from the ways in which he can be humiliated and he fearlessly proeeds to do so in this episode. Of course, he must know that when it comes to Angel, any humiliation can only be endearing in the larger context of the character.