I'm not really familiar with RE9, but a some surface level research suggests that (mechanically speaking) mutation is just "the abilities of bad guys change mid-fight".

That seems like it is easy enough to bolt onto more-or-less any game.

You just need a variant stat-block for the enemy and something to trigger it.

D&D 5.5 has a bunch of monsters with abilities which change when they are on half hit points but you can use other triggers such as "is the target is a spell" or "the GM decides the players are having too easy a time and spends a Fear point".

The trick is to come up with mutations and associated mechanics which fit the narrative.