I wasn’t aware there was such a thing as “rivals used to be challenging” rhetoric. But sure, if there is, it strikes me as probably quite silly.
Anyway, Blue. I think he’s clearly meant to be a jerk, because the whole thing with Oak turning up at the end of the game to call him out for not loving his Pokémon enough doesn’t really make sense if he’s not. Personally I think that, above and beyond just thinking he’s better than you, he consistently goes out of his way to be insulting to you and diminish your accomplishments. Like, I don’t know if he’s necessarily a bad person (well, I mean, he probably is, since he turns up at Silph Co. during the Team Rocket takeover and doesn’t lift a finger to help, but you could easily put that down to poor writing) but he always struck me as rather unpleasant to be around.