Mario is a character with no glaring weaknesses and no overpowering strengths. He doesn't hit hard like Gannon, but he's faster than him. He's not as fast as Fox, but he doesn't punished as hard as him. Mario doesn't have the best tools, but he has plenty of 'em. Considering this, you win with Mario through constant unflinching effort, and not through leaps and bounds. In other words, consistency is really crucial when playing Mario and you'll have to make sure that if there's something you want to do, then you do it without messing up.
Of course, like any character, you'll have to work on your basic tech with Mario first (shuffling, wavedashing, wavelanding,etc) and then work on some specific techs with Mario (getting full distance with his up-b, shabawd, learning your options out of walljumps,etc). You could also learn to up-b walljump cancel, but that technique is pretty hard to master and it's also quite situationnal, so you probably shouldn't start with that. After that, there's really not a lot to learn with Mario other than matchups and very specific stuff like we discuss here on this board.
Mario requires constant solid play. If you have that, you can probably become a good plumber.
