Reading the script without the intrusive ugly-factor introduced by Mr Perlman (bless him) made the similarity of the two characters more obvious than it was watching the film.
Also made me think - just how superficial am I? Seriously, I like Jayne, and I didn't like Johner (no more than any other character anyway). And there's only one difference between them: Adam Baldwin has a pretty face, Ron Perlman is butt-ugly (IMDB comment 'Frequently appears as characters who are deformed or not human'). Tragic isn't it?
To be fair I think there's a bit more to it than that. Don't you think it's more interesting if a character has characteristics that point in different directions (no, I am not scrabbling for justification)? And a lot of what I like about Jayne is that he acts like ugly Johner, but is unaware that he's a cutie-pie. Having a girl's name kind of rubs it in.