Is it fine to trim my hair 12 days post-op beard transplant?

It is best to ask your surgeon about post operative care as each doctor has their unique regimen.   In general most doctor would tell you to wait about two weeks.

I would suggest waiting two full weeks (so all the scabs/crusts are gone) before trimming but you should ask your doctor about it. If you really wanted to, at 12 days it would probably be fine to trim, not shave completely.

You are asking about trimming beard after a beard transplant and I generally tell my patients to speak with their doctor about such subjects