Mary Musgrove is seen as being a stupid snobbish woman who is hardly a guide to proper behaviour.
You're quite right in that money does not make you a noble. A title of nobility was originally usually given for prowess in war, and gradually became a reward for other types of assistance to teh king/government such as financial or political aid. Darcy has an estate worth £10k a year, and Mr Rushworth has £12k a year, but neither of them have titles. It is quite possible to be very rich, have a long lineage, (and we're told that the Woodhouses are rich and have a long lineage)and not have a title.