OP, to make Mega Man top tier, you must preach it, believe it, and fight for it. I do personally believe that Mega Man has marks to be one of the best characters, but since he controls rather unorthodox for a Smash character and comes with a complete kit no one would actually know how to use without a proper guide or lots of time in training mode, which is why I do feel it makes him a very underused character in the competitive scene.
Its not that he's really that hard to play as, its just the lack there of concise guides especially on video for those who don't have the attention span to read it is where lies the problem... exacerbated by the lack of use. Oh, don't get me wrong, I know that guides do exist for Mega Man, but Mega Man has evolved a lot over time the more tech being dug up, and when something new is discovered when its very unique to him, it normally goes unnoticed.
What would make Mega Man top tier would be all the tech he actually does have. Things like:
-The easiest jablock combos and setups that can lead into an infinite... even from a raw Metal Blade.
-DITCIT. which can also lead into footstool infinite and lots of other highly damaging combos.
-Falling Airshooter, which counters match ups casual Mega Mans would have trouble with in certain ways. Like Fox or Captain Falcon for instance get the smack down from this combo. Anyone tall or falls really fast gets owned by this combo, and if your character is tall as Falcon, its unblockable after shield has dropped low enough. (meaning it pokes) Does 40% damage maximum.
-Decent customs that don't overshadow his regular moves. Meaning they're all good when fully taken advantage of.
-Plenty of mobility and range.
That is what I think would make Mega Man higher on the list, if people noticed that kind of stuff and if said list actually mattered. I'm certain there would be people willing to learn Mega Man if they knew what specifically to practice.
Also since you asked @
Sneak Sneaks
here's that video:
https://www.youtube.com/watch?v=5bC24EGL8Nc