As good as MMH is, the character is pretty hard to use to max out his capabilities, and to take advantage of his tool set you need to make extremely good reads. The character is no where near broken, and honestly doesn't really have anything that is necessarily cheap in a game that almost everyone has cheap options. His 50/50s are within the realm of reaction, he has tricky set ups in the corner but he doesn't have broken damage like many others either. Also, like others have pointed out, many players still over respect him in many regards, especially when he doesn't have meter, and this just comes from lack of match up experience against him.
And the argument being made that he isn't being represented in enough areas to gather the match up knowledge to make him more dominant could also go the other way as well; limited representation can also prevent people from learning the weaknesses/counters to the character.
Also the limited number of people playing the character could also be a testament to how difficult the character is to play to be able to make his winning match ups winning or even, even; not everyone can just pick up the character and be a god without putting in a considerable time or we'd be seeing more MMH dominance.
I'm not disputing that fact that MMH is among the very top characters in this game, but when I lose to him I at least feel I was outplayed considering he has to open you up honestly, whether that be by making the right reads or his opponent not respecting his numerous options, when there are other tippy, top tiers that also do considerably well against the greater majority of the cast with very limited bad match ups that have cheap tool sets such as Batgirl that only need to touch you once to win a round, or Zod that can trait up and throw interactibles at you for free(not to say Zod is nearly as cheap as Batgirl, because he isn't in my opinion at least).
I don't know the numbers for his match ups but 6-4's are far from unwinnable so if the issue is that he just has an abundance of 6-4's and 5-5's with a few 7-3's and no losing match ups, I don't see him as the potential boogeyman that many are making him out to be. He's definitely a scary character to play against, being too well rounded, but no where near broken.
i respect the fact that he takes skill, as now a fully debout member of the church of martian christhunter (zod is the priest of my church), however i feel very strongly that martian might be as broken as kabal. I also think you need a very smart player to untap his potential with a solid fundamental base and more importantly both strong at zoning and rushdown. unlike other characters like batgirl and doomsday, actually batgirl can be a wicked good run-away sllow the game down type character
However the fact of the matter is:
1.He's ridiculously good on knockdown and waking up, he has so many fuckng options once he phase charges you tha the will open you up, especially with trait , he has cross up tp setups off the yin yang,
2. he has a normal in trait that is over head anti-airs multi-hit and hit/block confirmable to free pressure.
He also always has meter to extend his pressure.
3. His corner game is ridiculous, and builds shitloads of meter when he does it and chip. and his pressure is excellent.
4.his standing 3 is essentially kenshi's shoulder when he is in trait, but if they block it, you can get free pressure or they can't reach you anyways.
5. He can't be zoned.
6.His zoning is excellent, he can controls 2-3 points on the screen at once and can hitconfirm his zooning. Also his zoning does good chip and build sshitloads of meter. Not to mention he can stall if he needs to. And can switch back to rushdown when he feels like it. A goo dmartian makes it so that by the time you do get past his zoning he is going to have trait again, and you have to eat pressure for days, if you aren't patient vs martians zoning he will snowball the shit out of you
7.you can't throw interactables at him, and he has a multitude of ways to either get free interactable damage or take advantage of them.
8. along with his highly effective pressure, he has an air dash and also a meter burn airdash in case he feels breaking your block
9. he always will have meter
10. His back 3 is multi-hit so its busts al armor and it has good startup and reach
11. he has proximity mines
12. he can sandwich you between orbs and spread mayonaisse on your thighs
13. free interactables
14. green
15.even his back dash is effective