So, I've got a mutation on a themed pet. One I don't want. Is there an item to change it to a different mutation? Or am I just stuck with it?

The only ways to change traits on a themed pet is via
a) a Philter of Trait Disruption (Themed) (cheapest on the Trade Broker at the moment is 550k gold, also obtainable randomly via. Winter Wonderland while the Winter fest is on. This reshuffles a themed pet's Trait Points randomly) or
b) with a restricted trait disruptor for that species (full list of these here- only three currently available are the ones released for this fest, although there are others on the broker. However, these will only replace whatever the vis is with a random trait out of the four traits in that philter's set)

or you can use a philter of nullification, then use a genetic mutator
Note: only works on tamed sylestis

Thank you both! That answers my question

I would like to add that if you don't mind adding no new traits, you can just use a philter of nullification, and those work on untamed (bred, for instance) pets too- it's just generally not worth it unless you really hate that trait.