I can't really say I find any of them unattractive, appearance-wise, but a lot of them are definitely forgettable.
-Pryce: Most gym leaders have something interesting about them or some secondary occupation. Pryce is just...an old guy. That's it. Though I'll admit they made him look 10 times more epic in HeartGold and SoulSilver, that still wasn't enough to make him terribly memorable.
-Chuck: Not sure why, but I've never really been a fan of Chuck. He's got kind of an over the top "meathead" kind of persona, but somehow still doesn't manage to leave a lasting impression. Even with his creative waterfall gym in HeartGold/SoulSilver he's a pretty forgettable character.
-Grant: Another one who's really forgettable, which is a shame because I really liked his gym. And I can't help but think that he's got candy stuck in his hair.
-Giovanni: I know how everybody raves about how him being the Viridian Gym leader was like, an awesome plot twist. But personally, I thought it was pretty lame as far as plot twists go. And the fact that he's this big tough mob boss who keeps getting spanked by a 10 year old makes him a pretty lame character overall.
And finally...
-Marlon: His slang is pretty entertaining at first, but after a while it gets kinda cheesy. I mean, even people who speak the heaviest slang in real life occasionally pronounce the "g" at the end of an "ing" word. His slang just didn't quite seem natural.