Is there anything I can use on my starter pet to make her more attractive? I didn't know about all the neat genes until later and she's kinda plain xD :( are there any philters that work on starter pets? (Like color or mutator ones?)

You can use dyes to change the colours of your starter pet. Most of the other philters work on starters.
Mutators can only be used on pets that are caught.
Stat philters can only be used on hatching eggs.
Pies can technically be used, but there's not much point if it's already an adult.
You should be able to use gender changing philters if you want to change your pet's gender. But this works only if you have not bred it.
Philters of learning/unlearning work. it'll stop your pet from gaining experience if you want it to remain at a certain level (mostly if you want to fight in the arena)

I think that's about it. So the most you can do to change your pet is to dye it. Nothing much you can do about its genes/ mutations.

Alrighty, thank you!