Powerful Pokémon and cool Pokémon are important to have in any arsenal—but they're not the only kinds of Pokémon you'll use. Everyone needs Pokémon that are useful, to help you capture and train other Pokémon.
Series creator, Masahiro Sakurai, said in an interview that he was all but done balancing the game. Amidst the grumblings of fans, I kept seeing one joke about what this meant: that Diddy Kong was "here to stay."
Super Smash Bros. can have a pretty high barrier to entry compared to other big Nintendo games. If you're a newcomer to this Smash generation, I'd recommend watching this video tutorial from YouTuber blakinola.