Merlin is a not a girl's name in any sense of the word.
Merlin, being a well known MALE wizard and mythological character, is 99.99% of most people's association with the name. It's not like
Merlin is someone obscure-everyone knows who he is. I don't say this ever, but
Merlin on a girl is asking her to get her ass kicked. She'd just go by
Elizabeth, and hope nobody ever finds out her first name.
If you must use
Merlin on a female, use it on a dog or a fish that can't object.