I don't think anyone who was going to buy 5e is going to not buy it because there's a Flumph in the monster manual. That doesn't make it less dumb though.
Yes. Some of the early editions of D&D had frankly stupid and useless monsters in them. They were just throwing stuff at the wall to see what stuck. The game has since shed much of the cruft and no one even played the world's tiniest violin until OotS made a recurring joke about old stupid monsters from AD&D. Hurr hurr. The Flumph SO RANDUMB HURR.
That's all this is. It's a joke from a webcomic. The Flumph isn't even especially stupid or useless (it's just regular stupid and useless). It's probably not even in the bottom fifty stupid and useless monsters in AD&D. The Flumph does not, itself, tick me off. I'm not sure what burns my bacon more: The people who made it some kind of hipster-esque campaign to include the Flumph or the fact that the devs acknowledged and acquiesced to that faction. Don't get me wrong. I'm not against people liking what I don't like, but no one likes the Flumph, especially not the people who got it put in the book.