An important question

No, I don't think it makes a man 'less of a man' for crossplaying for fun. I don't see anything wrong with having fun and dressing up as your favourite character regardless of its gender, or how it makes someone 'less of a man'. So, no, it doesn't, in my perspective. Although, there are probably a fair few people who do think that, like your mum, but, who cares? You should just do what you want to do and not care what people think. =)

? Clothing does not have genders. Hair lengths do not have genders.
Boys can wear whatever they want.
Girls can wear whatever they want.
Nonbinary people can wear whatever they want.

It's not a matter of "boy clothes" or "girl clothes", it's a matter of "hey, i want to cosplay as a female character" And if you want to, sure! It's not right to judge someone based on the clothes they wear, no matter what it's for.

And "act like a boy"??? Dude, all people have different personalities and can act however they please. Gender doesn't have anything to do with it.

What I'm saying is:
Go ahead. Cosplay is supposed to be fun, so don't let others stop you from enjoying yourself. I'm sure you'll be a great cosplayer!