Why are you surprised? Age-gap yuri is an established conservative value.
Well like I wrote I've seen age-gap yuri before. Only cited one example there but I've read several. What I haven't read is a manga about a lolicon, female or otherwise, quite as "about to explode" as Mimika. A few factors make that aspect probably easier to swallow for most, like how they're both girls, or how Mimika's still a public school student (not for long, and there's still a 7 year gap of course), and the general lighter atmosphere, but a pedo's a pedo and the West always has its pedo scare (like, reviewing Kodomo no Jikan, which in some ways is LESS "iffy" than this since the protagonist doesn't actually like the main heroine that way for a long time got people worked up).
Personally I have no real boundaries for what I'll read aside from probably full on BL, because that's usually aimed very specifically at women, but I usually remind myself that I've also read a lot and thus have a lot less concern. Hence my surprise.