Mario is Mr.Fundamentals with good stats. If you understand anti-air you have his Usmash. For juggles he has Utilts and a basic throw combo setup. He reflects, he has a zoning projectile, etc, etc. His top tier stats let you expand on all these and adapt as you get smarter at the game and the competitive mindset without an autopilot/get out of jail free like some top tiers have.Never understood why so many think that Mario is easy to use. Perhaps execution wise he's kinda easy. Nobody ever explains why they feel he's easy to use though.
His neutral isn't nearly as dominant as the other top tiers (though ZSS's isn't that great either) nor is his disadvantage. If he was really as easy to use as so many have said I would expect to see more success with him as a secondary but I haven't seen it yet.