Finn’s not trans, he’s a he. He just cross dresses.
It’s fine if you’re weirded out, that’s a feeling not a refection of how you’d treat someone. If you’d treat him the same regardless of how he dresses or why he dresses that way, you’re not being phobic as far as I can see.
I think the contradiction between his appearance and voice/attitude is likely responsible for any weirded out feeling you have.