How would a fem mello and Near be?
Hmm...an interesting question!
I don't think their behavior/personality would change very much if their gender was swapped but other people's perception of them would definitely change a little.
I think fem!Mello's outbursts (however few they may be) would make her look kinda like a stereotypical emotional woman? And her rigging her base would be seen as needlessly paranoid since she's a woman and women's fears are usually not taken seriously. Her love for chocolate wouldn't be doing any favors to the sexist stereotypes others would have against her. Also fem!Mello's actions would be kinda less bold than male!Mello perhaps?
Fem!Mello may also have a bodyguard from the mafia for her protection. She would be respected for securing a spot in the mafia circle of hers despite being a woman. She'd had to have tried harder to prove herself to them than male!Mello did because DN characters are sexist like that. Oh! And fem!Mello would seem a bit like a gothic tomboy who's brave and a little out there.
Fem!Near would be kinda like Meteora Österreich my favorite character from the anime Re: Creators (this anime is sorta obscure but it's on Prime Video so...):
Anyways they're both intelligent and stoic characters (they even have the same hair color!) who speak in a monotone voice. Fem!Near would have long hair (like Near did in the oneshot) which she'd really like to twirl. She would maybe think that fashion is pretty stupid & go like #comfy clothes forever actually haha.