I feel like thunder is pretty useless in this matchup. Wario's aerial mobility is just too good to get hit by it and we might get punished for using it. Given that, I think we are really limited to three moves to kill him with until really high percents: fsmash, usmash, and nair. Like Kaylo already said, usmash is pretty risky because if Wario blocks it, he'll probably fsmash or grab us.
Warios always try to get behind me with fsmashes for some reason (maybe because of that darn good mobility they have). I've gotten to the point where if I'm about to space and fsmash to punish a Wario coming back to the stage, I turn around and smash the other direction in anticipation of him landing behind me.
Against dair camping I think our dsmash wins out. Someone will have to confirm or reject that because I'm not entirely sure.
That's all I can think to say that hasn't already been said. I think this matchup is annoying but winnable for sure. I know Wario has trouble killing us as well, but considering that we only have like 2 or 3 usable kill moves against him, I think he has an easier time. Maybe that's just the Pika bias in me. It's probably even considering we can knock him off the bike and gimp him.