Call it "filler", call it "fluff", call it "discovery channel writeup"...
...I call it motivation, lore, mystery, culture, habitat, and just plain old "background" that places the creatures within a world and not just within a dungeon/adventure/the end of a sword.
But sometimes this is irrelevant to the task at hand and there are some things that you CAN just look up on discovery channel. For example one of the first adventures in my upcoming campaign is about a man with an aberrant mark who can control insects, vermin and other creatures. He has assembled quite an array of giant spiders, giant centipedes, beetles and similar. The "fluff" that supports this is independent on the creatures I'm actually using, because the justification is from being bound and controlled by another individual with a powerful mark (Eberron campaign).
The fluff of the individual monsters is rather irrelevant to me after that point - their mechanics become paramount. I want to make fun, themed encounters that don't get stale and can cover a variety of different ideas. I don't need to know the complex mating rituals of centipedes and spiders (albeit, I already do know as I have a strong interest in entomology) I want "Centipede and awesome mechanics A, Centipede and awesome mechanics B" and the biggest variety of them possible. Putting in irrelevant fluff for simple monsters reduces the number of actually usable monsters without enhancing anything at all.
On the other hand aboleths, illithids and other things definitely deserve explanations. The more extraordinary the creature the better the fluff should be to explain what it does. The more weird or unusual, the more fluff becomes important just to get an idea as to what the hell it does. Do you honestly need fluff that tells you about a giant centipede is beyond a paragraph of its dungeon/forest habitat? Does anyone need to know why a giant spider lurks in dungeons and builds webs? On the other hand, something that's completely weird like a Nerra needs more explanation as to what it is before anyone will use it. There's nothing to relate the creature to or figure out what on earth it's about without fluff. That's where I agree fluff is absolutely essential.
But returning to the original point, fluff is not always essential but mechanics (IMO) always are. A non-compelling monster can be saved by mechanics. A compelling fluffy monster can be absolutely ruined by terrible mechanics. I don't need to know much about a giant centipede to want to use a giant centipede, but I do want to have a fun giant centipede monster. As I said previously though, I really need to know a LOT about an illithid not just for combat, but to understand its motivations, where it lives, how it behaves and such to make it genuinely unique as it hasn't got a simple real world equivalent to base it on (at least I hope not

But again, no amount of fluff would save an illithid if it isn't any fun as a monster to use*. I either have to make a creature or use one of the other numerous monsters that do both great fluff and mechanics (across all editions). At the same time, a creature can be all fluff and doesn't need mechanics at all. I disagree with you when you say the singing mushrooms need mechanics if they are distinctly noted as being non-combatants.
What point does HP, defenses and similar have against something that can't defend itself? In 4E I'd make them minions with a base 10 defensive line and that's it. I don't need a MM to write that to simply make a quick common sense ruling on the matter. Not that I would even go that far most likely either. As for the stone head, just give him DR 5 per tier and probably soldier defenses and I'm done. If he can actually fight back, then maybe I would give him a full stat block, but if he can't defend himself he has no need for stats. The PCs just spend an hour maybe hacking him apart and they're done with that, with whatever consequences in the story that causes later.
Why does it need stats? Because when I play a video game, I like being able to click on anything, not just the "set pieces" I am "railroaded" into interacting with.
Why does not having stats make something into a thing you can't interact with? This is rather nonsensical argument. Video games have the flaw that they make their NPCs invulnerable or you can't target them. Not giving a creature stats is not the equivalent to this in any manner, it simply means if you go "I magic missile the cat!" the cat just dies. Do we really need to care if the cat can dodge a magic missile or an acid orb or whatever else? Is it relevant? Personally I think the lord coming back to find her prized cat is now a bubbling pile of fur more interesting a consequence. If the cat could potentially scratch a party member for 1 HP of damage before dying in combat just doesn't seem relevant to me.
*Illithids are awesome and have been for ages. It's just an example to illustrate the argument - not highlighting there is anything actually wrong with Illithids in any manner!
I HATE the word fluff. It's terribly dismissive of the very material that hooked a lot of people into gaming in the first place.
I played Wargames before I played Dungeons and Dragons. It was Warhammer and then a game of 1E DnD, then 2nd edition DnD and then taking over from DMing after that point that got me into things. The first 1E and 2E DnD games barely used anything from Dungeons and Dragons that I can recall. Much of it was very much like playing a roleplaying game of Warhammer with Dungeons and Dragons, because we used the models.
Although the word fluff may be dismissive, I like to think that it's only being equally dismissive as those who think mechanics are irrelevant to a monster are being. Making a monsters background and ecology is one thing, making that a compelling creature that makes an exciting battle is the real challenge to me.
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