The problem with Mirror Mettaton is that he hasn't quite developed the patience for the long con as his real has. The sweet, slow delivery of a successful mental twist of someone's arm is not something he has learned to look forward to. He has all of the impertinence and impatience of a child, and it shows in how his face furrows in anger at how this just! Isn't! Working!

"You don't get it, do you? If you were perfect and had stayed to your primary function, your creators would never have turned on you! It's your fault they hate you and want you gone! Perfection was there and you cast it away!"

This is so open and blunt and not subtle at all, but if this is what it takes to destroy whatever good feeling this robot has towards his Real, he'll take it.