Yeah, of course. I mean it's really hard to teambuild on cart, because a lot of options are closed to you, but if you are able to build and create an effective, working team, you can definitely become a good player. It's just really hard, since the amount of competitive people playing on the cartridge is really low, so you have to make sure you find good people to battle with. Otherwise, though, as long as you can find away to effectively build your team and battle good players, you can definitely become good on cartridge :)
Just note, though, that the simulator is probably much better for improving your skills, as you can play a lot of battles in a short period of time. You're also more likely to find more competitive players, since the whole simulator is based around competitive battling. You probably already know this, but in case you don't, the Showdown! simulator can be found here
http://play.pokemonshowdown.com/
