Mario pretty much loses to every other top tier because, like I said, he has no real main strength while they all do. His disadvantages aren't huge and he just has solid stats all-around, but his advantages are never huge either. That's just not a top tier character. That's a high tier. Top tier characters dominate and have match-ups so obviously skewed in their favour (Rosalina VS Ness for example), which Mario does not have.
If Mario really, truly lost to every top tier and just had "average traits" I don't think two different players would have won incredibly stacked majors with mostly solo Mario. I don't even understand where you're getting this conclusion that he loses to "pretty much every top tier", anyways.
Claiming Mario has solid stats all-around is completely false. He has some of the best moves in the game (bair, uair, nair, dair, usmash/fsmash, dthrow, up-b) alongside notably not-"average character" traits such as top tier boxing, juggling, air game, and damage/kill potential.
Cloud fills the same role as Mario (being an easy to use character that anyone can get into easily), but is much superior to Mario in everything from his projectile, his range, his speed, his power, and in the fact he has Limit. His recovery is weak but Mario will have a harder time getting Cloud off-stage than the other way around. Cloud also has match-ups that are obviously in his favour.
Blade Beam is really bad and has just worse overall data/usability than Fireball. Cloud's onstage presence is hindered by his weak grounded game (poor throws+terrible grab, lack of a safe grounded poke, etc. His best move on the ground is just super camping.), something that Mario doesn't have to worry about as much (mostly due to his amazing grab game and strong grounded normals).
If Cloud really, truly were just straight up better than Mario he would be filling top 32s (he doesn't) and Cloud players would have won majors already (aNTi's pocket doesn't count).
Cloud's disadvantage is also a notable flaw that continues to rear its head. As Cloud counterplay continues to evolve, so does the edgeguarding. While Down Air is a silly, silly move, it isn't invincible and people are aware of this. Cloud is combo food and his weak recovery is continually exploited.
Cloud nerfs hurt a lot more than people realize; the roll weakening was somewhat notable, and the LCS nerf was a slap on the wrist, but the Up Air change was huge. Cloud lost a safe poke and a reliable killing juggle tool.
This is no different to how Rosalina is always ranked above Ryu on any tier list because, while more people play as Ryu and he probably technically has a better player base, Rosalina shuts Ryu down so hard it doesn't matter, plus a Rosalina in the right hands will do just as well against the rest of the cast, if not better than Ryu. So Rosalina automatically gets a higher position regardless of how she herself is doing in tournaments.
This is pure theory and doesn't reflect upon the rest of your post at all.
Winning/losing a specific and somewhat uncommon matchup (which he doesn't actually "get shut down so hard", you're gonna have to explain that) doesn't make a character better outright. That's not how tier lists work.
Like I said, Ally is just really good, and he put in a lot of work. Not anybody can just win with Mario. We cannot begin to consider Mario top tier until we see a bunch of newbies placing highly with him, or players of other characters suddenly switching to him and doing well.
Mario, from a purely statistical standpoint, was already everywhere in tournaments at every skill level. Just because he isn't winning tournaments or carrying players or whatever doesn't mean he isn't top tier.
I don't understand how "no one can just win with Mario" proves anything. What is that supposed to mean? No character can just cruise you to top 8 in this game's current state.