Generosity does not only apply to situations in where materialistic objects are involved, generosity means by all needs putting others in front of you and willing to sacrifice your own needs for others, and Rarity might not show this plenty, but I considered get episodes to always have the best lessons. "Sweet and Elite" and "Sisterhooves Social" both had similar lessons, while Rarity through out the episodes was somewhat selfish, the endings were what she learned out of it, so she does improve episode after episode. In "Sweet and Elite" she risked her social status for her friends, after going through the hard work of impressing others and living what she always dreamed of, but in the end her friends were far more important, and in "Sisterhooves Social" her annoyance with her little sister was something people with younger siblings can relate to, and she solved it doing something that not that many people would be willing to do. So, no she's not selfish, and her generosity is definitively there.
*The auto correct on my phone is sent from hell to make me miserable.