NAH
OP you are not the asshole. Your actions came from a place of kindness. Your mother is not an asshole either and she is not mad at you. She is mad at herself. She feels like she has failed in providing for you the way she always has in the past. No one has done anything wrong.
My suggestion: It may not be easy but try to respond to her irritation with kindness. Sometimes when people are the most stressed and the most in need of care they push people away. Maybe get together with your siblings and brainstorm some cheap/free activities you can do as a family. Then suggest that in a way that implies that she is being nice in letting you + siblings do that eg one Christmas when I was a child and cash was really tight we decorated the house with tons of paper snowflakes we made and then we went out and bought the ingredients for a nice dinner and then we cooked together and after the meal we went out skating in the local park came home and drank hot chocolate and watched movies. It was fun and TBH I remember that Christmas more than the toys of other wealthier Christmases.