I wouldn't call it casualised, which is a pretty damn vague term anyway. The newer installments are definitely not casual, and they are harder than Medieval 2 and Rome (which were pretty easy, really).
Arcadey would probably be a better word. The battles happen quite fast, and it doesn't really have the feel of a real battle. There's not much point in going for the historical formations, and the AI is a dumb-as-bricks as it always is (probably why the game is arcadey to begin with). All this, apart from the AI, can be solved by mods (I prefer R2TR).
The campaign is pretty good, though, and vastly more detailed and accurate than, say, the original Rome, which had all of Gaul as one faction and Egypt (just. . . Egypt).
Edit: Ninjad
I suppose you'd have to define casualization first as it seems like a rather vague and broad word.
Simplifying the mechanics of a game and/or reducing the depth of gameplay and "what variety of things can you do with this game".
In that sense, no. The game is arcadey, yes, but the mechanics have definitely not been simplified or lost depth. The formula has been changed, some things were made less of a pain (army management and recruiting), some things were cut (family trees and characters with character
), and quite a few things were added (tech progression), but it's still a Total War game.