Underlined text is entirely too general and incorrect. It's true that RD isn't the embodiment of a good pony, but that's what's great about her. RD gives the Mane 6 depth. Her and Rarity are arguably the most self-obsessed characters on the show. The thing is, everyone is flawed, some more than others. RD is an interesting and essential character because we have to watch her grow and go against her typical personality traits so that she can learn to be a better friend and better pony. She is learning from her friends what it means to be a better pony.
I don't love Rainbow Dash, but I think she's the pony that really rounds out the group.
ETA: I also don't feel that "competitive" is a bad trait, in-and-of itself. Competitiveness, when taken too far, can be a bad trait. I feel Rainbow Dash shows how this can be a bad thing in "Fall Weather Friends". However, as a general trait, it's not necessarily a poor trait to have and RD does not consistently go overboard with it.