I realize Jonsa is very popular with a lot of fanfiction written for the ship and then when I come across one of those it's just different iterations of the Jon and Arya relationship that GRRM wrote in the books. And hence the undermining of Arya as just a Sansa side prop and Jonsa cheerleader, not good looking, only good for fighting and sailing far, far away from her brothers.
I just came across this fic on the tag and was bored enough to click on it and it has all the hallmarks of the typical Jonsa fic in terms of how Arya is written. Jon says all this about Arya -
Note that none of this is canon to the Jon-Arya relationship in the books. No, Jon doesn't think Arya is running around biting the serving people - she was friends with them. Jon doesn't think Arya is 'dangerous' - he is worried for her because she's just a little girl. Jon knows Arya is the one who is hiding from the scary Septas and mothers. Jon thinks Arya is pretty. Jon doesn't think the Starks are ugly or plain or not good looking. Jon has also mocked the appearances of the children of great houses including the Lannister kids.
Unfortunately, poor Arya HAS to be mischaracterized in any Jonsa fic - and considering how much Jonsa fic exists, one can imagine why there is so much sexism surrounding her character and why she has been misrepresented so thoroughly in general.
Getting back to how much is taken from Jon's canon relationship with Arya, take this trope that is incredibly popular with the Jonsa ship -
They love Jon Snow being mindlessly devoted to Sansa and doing her every bidding. And yet we have another devotion in the books, in canon, where Jon Snow is breaking oaths sworn in front of the Old Gods - oaths he holds in high value and held for 4 books - to try and save Arya Stark. While he didn't spend an iota of a second worrying about Sansa.
Here's GRRM writing an emotional goodbye scene for Jon and Arya -
Meanwhile the Jonsa shipper writing Jon talking to Sansa -
GRRM about Jon and Arya being each other's home -
Jonsa shipper about Jon and Sansa -
GRRM about Jon and Arya being close -
Jonsa shipper about how Sansa was Jon's favorite of ALL his siblings -
GRRM about Jon and Arya being brother and sister -
Jonsa shipper about Jon and Sansa being brother and sister -
GRRM writing Jon describing Alys in a magical way with her sparkling eyes and flushed cheeks, comparing her smile to Arya's and admiring her bravery -
Jonsa shipper then disparaging Alys' (and Arya's looks) -
I love how they had to put that distinction in there that Jon loves Sansa's Andal high cheekbones and delicate noses and not these Northwoman with their long faces and that he doesn't desire anyone who looks like Arya!!
Beauty and appearance seems to be the central theme of pretty much all Jonsa fic including a paragraph somewhere mentioning how Arya is not good looking compared to Sansa.
Take any Jonsa fic and this is the pattern. Sansa is beautiful, the best diplomat and powerful politician. This is how the stan sees her and hence this is how Jon Snow sees her. This is what the Stan thinks is important and hence this is what Jon Snow appreciates most. And then add in some bits in there about how Arya is the opposite - running around biting people. Rinse and repeat.
There's more but yeah, I think I have covered the gist of this. What I am getting, again and again, from Jonsa fanfic, Jonsa metas, Jonsa theories is that this is a bunch of Sansa stans who desperately wish that Sansa had Arya's relationship with Jon Snow - from the very beginning - and then pretend as if they like Jonsa better because Jon and Sansa are not as close and therefore work better as an incest ship.
When in reality, they covet that close relationship for their fave and this is the primary reason they character assassinate Arya (and Jon) so much and assign made up character personalities to her.