This is one of the worst things you can hear from someone else in the game (aside from "You're cheating" or some of the strings of obscenities) because it's not just rude, presumptuous, and self-righteous, it's also outright wrong.
There is no "right" fluff for an army. A lot of people get up in arms about things like being able to field a ton of Purifiers or having Grey Knights and Daemonhosts in the same army, but I think that's very wrongheaded and harms the hobby as a whole. Maybe my Purifiers or Daemonhosts represent something else; or maybe my GK are part of a secret order of Purifiers that keeps itself hidden from the rest of the Imperium and thus aren't part of the small contingent that is name in the GK codex. Or maybe I just want to play some damn Purifiers and stop having a hissy fit, you big babby.
Everyone has a right to dictate their own army fluff; that's part of the great thing about 40K, because it's set in a universe so fantastically large that even all of the cannon fluff we've seen so far doesn't even scratch the surface of what the Imperium contains. There are TRILLIONS of people in the Imperium, and a thousand (or so) Marine chapters; I think there have been maybe a hundred named planets and a similar number of official chapters. There is room for whatever wacky idea you see fit to put down on paper, if you so please.
So why is it some people insist on telling you what is and isn't "right" for your army? If I like Thunderwolves, or Purifiers, or Trygons, or Razorbacks, or Nobz, or whatever thing I have focused on, who are you to tell me that I'm violating the fluff? Sure, most Space Wolf bands probably aren't dragging around fifteen giant mutant space puppies, but who knows, maybe one of them is? It's not like those guys are anal-retentive like the Ultras are. Maybe my Guard regiment consists of only the best of the best from several planets who have been organized into an elite human strike force to complement the usual "overwhelming numbers" style of regular Guard regiments? Schaeffer's Last Chancers, anyone? Don't tell me all Veterans is "unfluffy," because that's bullshit; anything can be fluffy if you want it to be.
I'm not saying you have to like it; it's perfectly understandable if someone else's view of an army doesn't mesh with your own. One of the Marine players in my area thinks all-bike armies are stupid and shouldn't exist, even though they're written right into the Codex Astartes. And that's entirely within his rights, too- you aren't required to like anything. But try to exercise a little bit of tact when someone else brings an army that doesn't suit your personal tastes, okay? I totally don't get why anyone would want to play Footdar because I think it's about the dumbest thing in the world, but it's their choice to do so. If someone spends the money and time to make an army, let them; it's not yours or anyone else's place to dictate the army they want to play.
And yes, not everyone who plays an army will have a deep and extensively detailed backstory for it; that's fine, too. Some people enjoy doing that, some people don't. Some people like to name their models, some people just wanna play Future Chess With Tanks. Neither way is right or wrong, they're just different from each other. If someone asks you for advice on how to make their fluff more interesting- or their list more competitive- sure, go ahead and give it, but don't burst in unasked and inform them that their army is bad and they should feel bad. Let them enjoy this hobby as they please, just as you do.
