Okay, to both of you. SSB =/= pokemon main games. Don't even think of judging one pokemon's performance in SSB based on main games and vice versa.
Lucario is good in pokemon games because of his versatility, really nice stats and also good resistances. In SSBB Luke is difficult to play, as he has to rely on racking up damage, because in early game he is somewhat weaker. Also winning positions with Luke are making him only 80% strong, which isn't nice at all.
Pika is awesome in SSBB, yet in main games, it's kinda suckish.
Anyway, in SSB charachters are adapted to the game so that it won't be unbalanced. If things were unadapted, then I imagine psychics rule the games while swordsmen and other simple fighting guys staring at their mightiness. Think of the same Mewtwo: he'd just go and use his psychic to crash everything (baibai Link, Marth, Roy etc). He had to be nerfed so that he would fit the game and everything would be (more or less) balanced.