Even easier to stop. He has less range and a worse grab range. Jabbing flat out stops him in his tracks and once you're passed 80% he's flat out pathetic bottom tier garbage, he has weaker moves than default with less range.
Let me state this again, the character has no air kill moves that don't send him into a helpless state, none of his grabs will kill you themselves and he has no grab follow-ups past 70%. His F-smash is a pile of garbage that hits on only frame 17 while having an insane amount of cool down. His U-smash is a weaker Fox u-smash with less range, and his d-smash is just weak.
As a Mii Brawler main you have to literally be ON POINT with getting the grab at just the right percent and then not screwing up your combo. Mii Brawler seems powerful because people don't realize how much of a small window he has to actually kill you early, but if you understand what percentage he needs you to be at, you can avoid him until he hits you with an air attack or something that puts you outside of the the % range he wants, then have fun because he's no longer killing you. Just shield a lot and bait out all the Up-B OoS he'll do since its his only semi-decent kill option at that point, if he grabs you, who cares? I already stated he can't kill you with a grab at this point.
Don't get me wrong, this character will DESTROY you if you don't know how to fight against it. But the moment you realize what his gameplan is and how easy it is to screw it up, it becomes a lot more manageable. Listen, just up until recently people are figuring out that DIing up is bad and Diing directly towards the stage horizontally is good. People were dying to Helicopter Kick FASTER due to BAD DI.
Have many people figured this out yet? No
Can Mii Brawler still be good if the player himself is good enough to get around these weaknesses? Yes
As a Mii Brawler main myself I've came across people who have understood my strategy well enough, they give me a lot of trouble.... so I understand my characters weaknesses. So I do whatever I can to counter-act them as well as come up with solutions when things don't go my way. It's what separates the best Mii Brawlers from the average ones.