Helicopter Kick is the dumbest thing. That's pretty much my only assessment on Mii customs lol. No reason you should be dying at percents like that without any rage.
I feel as though the actual problem with Helicopter Kick isn't just that it hurts quite a bit, but rather, that it is easy to "XYZ" into.
"I hit you with Fair 1? Let me just XYZ into Up B for a KO."
"Hitting you with Fair 1 isn't enough to KO? That's okay, let me just Fair 1 into Fair and then Up B for a KO."
"Oh, you're trying to jump above me now? That's okay, let me just Uair to stuff your jump and then Up B for a KO."
"Never mind, I can just cut the middleman out. I'll just wait for you to mess up and jump/airdodge above me, and then Up B for a KO."
"Oh, you're shielding now? That's cool. I'm gonna run up to you, Dthrow, and then Up B for a KO."
"Uh oh, I made you aggressive and you're trying to approach me. Good thing I have this shield button here, so that I can OoS into an Up B for a KO."
"Oooh so you're now waiting for me to throw out Up B so that you can punish it? That's okay, I am actually secretly a hand-me-down Mario with good frame data and decent combos, so I don't even need Up B. I'm just going to keep poking you with my awesome aerials and easy damage conversions off of stuff like Fair 1, Dtilt, Dthrow, Uair, or even Utilt which has a weird hitbox in front of it for no reason. I'm gonna keep damaging you, and then wait for YOU now. And when you have a bad landing option, or just simply mess up, THEN I am going to Up B you for a KO."
Mii Fighter by himself is a combo-oriented character with great mobility, good buttons in the air, with decent throws, tilts, and a good Dash Attack. He also has a really unfortunate time recovering (with the 1111 set anyways), as well as an equally unfortunate time KOing (again with the 1111 set). However, Mii Fighter with Helicopter Kick becomes a bit...degenerate. Not just because of Helicopter Kick being a very strong move in general, but also because it is far too easy for Mii Fighter to play around with and set up into. It becomes an over-centralizing aspect of the character and how he plays, which makes him more boring to both watch and play against, even if it does make him more effective overall.
The other customs that the Miis have aren't nearly this bad, even if Piston Punch can be a bit silly sometimes. Although Piston Punch actually has some real drawbacks to it, such as making his recovery worse and not being as potent as Helicopter Kick, even if you can still XYZ into it for KOs. In any case, it still makes him a more interesting and dynamic character by comparison.