Personally, I don't mind bikini/swimsuit shots as long as they aren't sleazy in nature. I think that H!P's PBs tend not to have a lot of sleazy-type pictures in them - some that might be suggestive, perhaps, but you certainly won't find an H!P member pic with the member, say, wearing some ultra-skimpy thong bikini while covered in chocolate syrup in strategic places. That type of photo would be rather squicky, especially if it was an underage member like Koharu. However, fortunately H!P doesn't do that sort of thing.
I'd kind of like to see Koharu do a photobook "in character" as Tsukishima Kirari, actually...wearing some of the performance costumes from the series and other Kirari-esque cute clothes.