I'm under the impression that she's a subversion of Robin Hood. Even then, if she cares so much about the common folk then surely donating a bit of her money wouldn't hurt?
First off, you're "under the impression".
That is an assumption, based on your perspective of her.
How do you figure she has a lot of money in the first place?
What, you think the Red Jennies are some secret elite group behind the scenes, stealing from nobles and creating this secret little rich empire or something?
When we meet Sera, she has a haircut that she did herself with a knife, and it is always brought up about her "mustard stained outfit".
Do you really think she has that much money?
Now, judging by your blatant hatred for Sera, you would not be aware of a certain conversation you can have with her, as you can only have it with high approval. You earn her trust and ask her about her life.. she doesn't go into real detail, but she does explain that she's had to travel around a fair bit.. and was given a chance to run with the Red Jennies.
This, to me, doesn't spell out rich.
Not sure where you get this idea of being loaded with money, but I'd say it has something to do with an intense hatred, thus creating a situation where you read into everything and draw conclusions based on your fantasies or ideals.
She cares about the common folk, but it doesn't necessarily have anything to do with money. You can do good things for people, without having to pass over a dime.