A reputable piercer with clean equipment will not modify the body of an infant.
To the OP, do I think it is cruel to pierce your baby's ears? Yes, selfish also quite frankly. You aren't thinking about your baby at all just your own feelings about her being mistaken for a boy which is definitely not one of the better reasons I have heard for making a baby deal with that. You obviously haven't heard a baby screaming after a vax because to them it is no small prick. I know it is very normal for a lot of parents to pierce their babies ears but I find it disturbing that a parent would cause their tiny baby pain like that out of pure vanity. The kid doesn't even have a say in the matter at that age! I think like other PP have mentioned it is at least nice to let them be old enough to decide...
This is coming from a mama who has had her ears pierced 5 times throughout my life (all just single holes that have closed). My ears the first time when I was 5 were so infected because my mom wasn't completely on top of it that I had to go to the doctor to have them removed (the skin healed around the earring because I didn't turn it)...What if you accidentally let your babies ears get infected. That can become very painful.
Sorry to be so blunt but I just think it is wrong for parents to do this to their baby girls. I am not against ear piercing or any piercing having had a tongue ring for many years just not on a baby! I know this is going to come off horribly but I just can't not pipe up about this.