Discrimination against fishmen/mermen does NOT give Camie a sad past. She's relatively young and, judging by her attitude above land, it didn't seem like too great a concern for her. Papagg was the one who was concerned about Camie. Besides, discrimination started way before Camie's time. It's not her flashback to have. Also, if you were to use that as a flashback, then ANY fishman or merman is a possible nakama.
Also, helping her out with their treasure doesn't mean anything, either. They took time off from their adventure to help Vivi since she was their friend. They did the same for Conis and, well, anyone who he became friends with or had respect for. Coby, Mayor Boodle, Broggy/Dorry, etc… They help people they meet at their own risk/loss ALL THE TIME.
As for swimming fast, the same can be said for any merman/maid. I think it's much more valuable for them to have a nakama that could actually walk and hold their own in battle than one who can swim fast.