I started another playthrough and I've been playing for a few hours now, I"m traveling through Velen after 100%ing White Orchard. In all that time I have not seen a single red, blue, or green mutagen drop. Not one. I have a mutagen from the noonwraith and the griffin, but no regular mutagens.
I'm starting to think it's just not extremely bad RNG and that something might be wrong somewhere.

I've had playthopughs were it seems I get mounds of every muatagen except the one I need, and I've had playthoughs where I'm drowning in them in general. I started a playthough last week where I went about 3 hours without getting a single one, including the noonwraith and the griffin. Finally got a regular blue mutagen from a nekker, and that's it.

I never look through the changelogs but I wonder if their drops have been reduced in frequency.

First of all, red mutagens are extremely rare. Second, all mutagens are random. The best enemies to fight for mutagen drops are Nekkers, but not in White Orchard. If you're after specific monster mutagens, they are also random except for important quest targets.

I've started getting mutagen drops now, and have finally crafted a large blue mutagen. They seem much rarer than my first play through where I was absolutely swimming in them. I guess maybe I was lucky/am unlucky this playthrough, or the drop rates got changed.