Fluff and Crunch isn't a very useful dichotomy.
Of course, there's a lot of grey area in between, but I have more of an issue with what's implied.
Crunch sounds good. It's crunch time! This is the important stuff. Let's crunch on down to business.
Fluff sounds bad. What's all this fluff taking up space? Nobody wants to be concerned with fluffy things.
I'm not taking sides. The truth is most games benefit from having both. So I'd rather look at it this way.
Bones are the very core of what makes a game a game. That cheat-sheet you made for yourself that just has the bare rules of the game? That's the game's skeleton right there. D&D has HP, HD, Levels and Ability Scores making up the core of its skeleton.
Some games are nearly all Bones. Some games barely have a skeleton at all. When you start messing with a game's Bones you might improve it or create a clumsy abomination. At some point it will resemble an entirely different animal.
Meat is what lies on top of the bones, connected to the game's skeleton. This isn't meaningless fluff. It's part of the game. Still, it's stuff that can be moved around, toned, bulked out. Spell lists are meat. Classes are meat.
It might be hard to imagine the skeleton of a game without any meat at all, but it's entirely possible. Most players will have a preference for just how much meat they like to see. Some may want just enough to cover the bones, others will want a colossus of flesh.
Skin lies on top of the meat. While the meat may be acting as muscle, working alongside the skeleton of the game, skin is clearly separated from the bones. This is the map of the world in the back of your setting guide, the chunks of flavour text and the beautiful art.
You may call this fluff, but that does it a disservice. There are people who aren't interested in buying a game with a skin, just the meat and bones. They'll end up putting their own skin on the game.