Saying side b is bad is one hell of an overstatement, sure it's punishable, but it's still good. It's kinda like saying marth's fsmash is bad cause you can punish it pretty easily after he whiffs it, doesn't stop it from being an amazing kill move with a good amount of coverage.
Anyway lol, yeah all you have to do to deal with the jumping portion of side b, the one that starts from the ground and then he jumps out of it, is not jump, or to just throw out a hitbox more disjointed than his, which isn't hard cause I believe the hitbox for that portion of the move is the same size as the middle portion of his grounded side b hitbox, could be wrong though.
He doesn't have bad movement, but the problem he has is that he has to jump to get anything out of it, and unlike jiggly, he doesn't have multiple jumps to prolong his midair movement shenanigans. He also doesn't have big hitboxes, so he really can't advance forward with a meaty hitbox unless he's facing away from you, in which case he'd be using bair, but even bair isn't that big, nor is it something you really want to approach with imo. Wario's game plan is to pretty much drift around you to get you to flinch and whiff something so he can get his amazing punish game going, so don't take the bait, throw out far reaching hitboxes with low endlag to limit his space, and play super left right brothers, and he really can't do anything about it
So yeah basically what Umbreon said, but I used more words lol.