Fast Fireball, Gust Cape, and Scalding F.L.U.D.D. are probably Mario's only good custom special moves.
Fast Fireball's increased speed makes it easy to spam; Gust Cape offers windboxes, along with reflecting properties; Scalding F.L.U.D.D. can actually be used offensively.
Super Jump Punch's customs really aren't usable, since Super Jump's recovery really isn't any better than Super Jump Punch's, while Explosive Punch hinders Mario's recovery with its weaker travel distance.
Shocking Cape.
Disjointed kill confirm out of d-throw is godly, and it's a solid edgeguarding tool in its own right.
I honestly find Gust Cape overrated. The windbox is good for gimping certain characters with predictable recoveries and underwhelming airspeed (Mac, Doc, Ganondorf, etc), but you can get much of the same effect with default Cape if the opponent can't reverse their trajectory or commits to a side-b recovery, and I find the smaller hitbox for reflector is more hassle than it's worth.
Scalding FLUDD is another disjoint and it confirms into grab. I don't need to remind y'all that Mario has a hella good grab game.
Explosive Punch is definitely usable if you're savvy with it, you've got another solid kill confirm out of d-throw and Mario's jump height and airspeed means he doesn't necessarily need the extra momentum from default Jump Punch. It's probably better on Doc, since it's not like he gets much benefit from Super Jump's extra vertical recovery, but it's not something I'd discount for Mario either.
This is just based on my experience labbing Mario and observing others in a customs-on meta. Mario has insanely good customs if you know how to use them.