Yeah, the Groudon comparison was pretty weak, but it was the biggest Pokémon that isn't less detailed than a balloon (aka Wailord) that I could think of.
Thing is, the 3DS isn't weak at all, and it'd easily be able to carry a free for all with 4 Ridleys, as long as you make him about Bowser's size. Ridley really isn't very complex in his visuals. Heck, the 3DS has even had Ridley on it before, in DoA: Dimensions, and while he didn't have too many animations it could still carry a cutscene of him smashing the players into the wall, not to mention that he was a lot bigger than he'd be in Smash Bros.
Ridley's model won't cause any problems for the 3DS, the system is so criminally underrated around these parts, it's almost impressive how much it's being looked down at. The only thing it might have trouble with is exactly programming things, and even then it isn't all that restricted (which is why I consider ICs and PT to be in 100%). In terms of graphics it's not lacking at all, it just needs to optimize the models for usage on the 3DS, that's all.