To expand on my previous post, I'm all for as many weird D&D monsters as they can fit into the book. Mythological creatures are fine and dandy, but one of my favorite aspects of D&D are the downright weird creatures. Gas spores, shriekers, green slimes, lurkers above, rust monsters, flumphs, otyughs, pretty much anything that makes people wonder why the hell those things would ever exist comes of as particularly charming to me. I wouldn't want to use them excessively in a campaign or see them all in one Monster Manual, but they are a part of the game that makes D&D what it is and not just a generic fantasy role-playing game.
As to the flumph specifically, not every creature needs to be deadly and serious. I'm all for putting a couple of whimsical creatures in the Monster Manual that leave the players scratching their heads when they encounter one.