I main Mario, and all my input comes from watching high level players at tournaments, or playing FG. I don't think Mario is as good as the best characters, but that he's not much below them.
Mario's combo game gets severely marginalized by Sheik, Diddy, Ness, Rosalina and some other top tier characters. I think this is what keeps him below some of those players. When he can't land big strings or combos he becomes less elite for sure.
Another one of his flaws is that he lacks a kill move that doesn't involve securing a hard read, except maybe down-smash, which is still a tough kill move because of the weak knock back. Most of what I'm saying here comes from watching elite talent though, and not from personal use.
At my level, playing FG until I find someone better than me, Mario is fine in almost all match ups. I don't find many high quality Diddy or Sheik players, but when I do, I don't really feel overmatched. Ness can be a tough match because of the back throw, but it's still a match I've won against good players, and seen tournament players win.
Mario is really versatile, and doesn't really have a particular match-up he can't win, and as the meta game continues, I can see him moving up in tier lists. The more characters are considered tournament ready, the better I think Mario will be.