They are probably not thinking "wow that guy stutters, what a freak". Much more likely they are thinking "woah, that guy stutters but he isn't letting it stand in his way, he's up there doing his thing without even caring about it, that's pretty impressive". I'm totally serious. I don't stutter (just passing through) but that's what I thought the first time I attended a lecture where the lecturer happened to have a prominent stutter. I think that's a normal person's reaction really.