Well having an uninteresting, non-character main character I don't think is a good thing even if they aren't the draw so to speak. With regards to the Prison School comparison, I only bring that up because its a manga that is lauded for its art. Like with any visual media like comics, animation, games, film, etc. we all have and likely will continue to consume products we can all admit are critically flawed (bad writing, bad characterization, etc.) but still like because the art/animation is amazing. For as shitty a game as the Order 1886 may be, it still looks great. Monster Musume, IMO of course, doesn't even have that. As I said before, nothing about the art style stands out amongst the sea of similar titles. The art and designs are all super boring/"safe". 10 minutes on Deviantart/Tumblr return stuff like:
Obviously if these were for a monthly manga you'd have to lose the color and maybe simplify all these designs a bit more but those are all WAY more interesting and have more personality than what you actually get in Monster Musume - I'd probably actually read that hypothetical manga just for the art from any of the examples I posted.
Note, I'm not trying to throw shade on those who do like it. People can like things that I don't like. I personally don't think the manga has that many redeeming properties after taking a bit of time to look into it - the fact that it seems popular is just weird to me.
A LOT of Monster Girl art , even very good art, is NSFW. Made getting examples a bit of a pain :/
those are all ugly tho C: