It's true that Mario has Cape to answer our Sun, but it's important to keep in mind we have Sun to answer his Fireball, so at the very least, you can dissuade a projectile based approach in this way. Plus, like in a lot of reflector MUs, just keeping the thing charged to force the opponent to flinch with their reflector is also very important. If you are tricky enough with how you handle your Sun, neither party is going to be shooting very many projectiles at each other.
In the end, the matchup comes down to spacing Bairs with each other. Just keep short hopping around with your back facing towards him. If Mario tries to be too aggressive and advance with an aerial or dash attack/grab, try to space it out and plant your feet in his face. If he advances while you're not in a position to control him with a Bair, I heavily recommend getting the heck out of there with aircamping and platform camping. It can be obnoxious to give up that much stage control just because Mario decided to push some buttons, but Wii Fit isn't really a character who NEEDS stage control to function properly. While retreating, Mario is very likely to try to chase you down with Uairs, so make sure you throw in some airdodges (or Bairs if he puts himself in position for it) as you look to land on a platform, or just straight up veer all the way offstage to the edge.
For edgeguarding Mario, it's true that it's almost impossible to get him if he's in range to just UpB the ledge. That's why, when you knock him offstage, you have to reach him BEFORE he gets into that range. In general, you want to force him to airdodge, and then time one of your spikes or Fair. It is very difficult, but not impossible, and it's very satisfying when you land that business.
When it comes down to it, all aspects of the matchup for us involve watching for what Mario is about to do, and retreating a great deal if we know we can't really do anything about it at the time. Just be patient, and wait for your opportunities. I don't want to say this is a -2 matchup, but it's definitely not a -1. -1.5, I guess, then?