I'd compare the mushroom sisters to Doflamingo from One Piece - their powerset doesn't suggest they'd be a top-tier physical threat, but yet they are. They feel like overpowered enemies, which would be fine if they were the final boss but they're not. we know they're not. So instead, it makes them feel like time-wasters, especially when the fight doesn't show any sense of progress - there's no "trick" to figure out (as you would expect from these types of enemies) but instead they just need to get beaten down repeatedly until it sticks. It's been 7 chapters since they changed the battlefield, and 13 chapters since John started fighting that one (and Engi joined in a few chapters later). It took 8 chapters to beat Hercules, starting from the first fake-out fight. that's a LOT of pages spent on minibosses. And of course, unlike Doflamingo (who was built up a helluva lot more) these ones just sort of showed up at the end after only serving tea in the background. I think if they had established up-front something to give them credibility, maybe John going "oh the mushroom sisters they're one of the strongest there is! and Kyou has made them his maid? sasuga" it would've helped.
That said, I thought they were cute and even if it's inconsistent, the variety of mushroom attacks they had was nice. They can go hang out with En from Dorohedoro in the "overpowered mushroom mage" club.
...also it's pretty weird how the one sister got taken out by randos, but the other two were going toe-to-toe with the strongest of the heroes. I can accept a sibling trio where the eldest is the only actually strong one, and the others are weak, but it's weird to just have 1 weakling.
.......also the musical demons showing up to stall for time was pointless, they disrupted Clione so that Yumeji could get pushed even further to the brink, but if the ultimate finish was just "Merry recovers and wins" I feel like the whole fight could have been plotted better.