I think she started to like him before the serum. She was often impressed by his character during his training and is one of the ones who pushed for him to get the serum in the first place. His changed appearance after getting the serum helped, I'm sure, but it wasn't the deciding factor. It was mostly due to his character and the amount of time they spent together.

In the Agent Carter TV show (which may no longer be canon), after Steve was presumed dead, Peggy developed feelings for a handicapped man who also had a strong moral compass, like Steve did. If her priorities for attraction had power and appearance at the top, this wouldn't have been the case.