It'll cost you the same to keep this cow, and then to keep her and her calf, as it would a better cow. And you'll get a lot less for her calf than one from a better cow as it will inherit her poor conformation.
Assuming she's under 30 months at the moment, you'll get more for her in the fat now that you will if you breed her and then decide to send her away.
So yes, I would sell her and buy a good heifer.