Well, we need to distinguish two different types of off-model:
Style/realism off-model, and anatomy/body build off model.
The first happens when you take, say, Marceline and give her more defined muscles and bones, a nose, sclera, shade her body and whatnot. I like that, I'm all for drawing various characters with different levels of realism.
The second type is where you take Marceline and give her gigantic tits and a wide ass, or make her black, or white, or buff, or whatever. That I'm not a big fan of. Like, I tolerate it, but the less a picture resembles the original character conceptually, the less I like it. With art I save and masturbate to
I only allow a small degree of build-wise deviation.