It's not really that, it's just shields are supposed to beat projectile spamming characters (what Mii Gunner is). So if you give that character heavy shield pressure (Down B 2 is the biggest culprit by far) then you're taking away the one weakness that the character has.
With customs, Mii Gunner is easily high tier, maybe even borderline top tier, and it's a character that just outright isn't fun to play against, like Luigi and Sonic and all those other awfully designed characters.
Why should I have to play a character with a reflector to shut down a character; I don't understand.
Then you oughta perfect shield everything then, haha.
Besides you don't need a reflector to beat her, it's just really nice to have. You just have to stop her from moving. If you limit her movement options, she can't wall you out while she's cornered forever. If she's stuck by the ledge with no real way to get out of it, she's in a super bad position. Especially since her only safe approach option are Grenades and Fair, the former disappearing when clanging and the latter doing prwctocally no shield damage and is punishable if she lands while using it. Bomb Drop has too much end lag to be of any real use if the opponents shields or clangs with it.
Which leaves the side special options, most people run missiles that are slow and can be perfect-shielded easily and Flame Pillar I suppose is the only good cover since its main problem is just the end lag but the projectile gets the job done.
Either way, if you actually play Bowser you'll generally armor through pretty much everything anyways if you just dropkick your way towards Gunner lol. And don't get me started on how unreliable her kill moves can get.
Is there potential? Definitely, but I don't think she'll be dominating the whole cast. If you give people time to learn the matchup, I assure you she won't be top of the line material or anything lIke that.