Mario sucks against m2, marth, and falcon. Other than that he's pretty even on matchups. There's no mario specific tech you need to know, and conbos kinda happen on the fly. He's got an amazing grab game but poor grab range. As for stages just counter pick your opponent
Basically this. I think he's not hot against Ganondorf either due to various changes and the fireball nerf, along with Ganon buffs. It's basically reverted to the Melee MU to an extent, probably not as bad but still not in Mario's favor.
Other than that I don't think he loses too hard, and his new D-Throw Base Knockback actually lets him combo spacies better so he's more favorable versus them now in comparison to 3.02.
As a 2nd main or even a pocket character, Mario is great. Hell, you can even main him outright and have one secondary for any possible bad MUs. But if you're investing in him as a secondary it's an empirically good choice as far as this game goes.
A list of stuff to learn with Mario: Wavedashing, good movement because Mario has a plethora of options with that, utilize Dtilt as it's way better than Melee here, chaingrabs for days on certain chars, DSmash is amazing, so is FSmash, Set up into Fair combos/strings. Fireballs aren't great for camping but they can enforce good stage control if used properly. Just have good fundamentals and you should be a-ok!
