Being that you bought her shortly before she calved, and that she was a bit on the thin side, I might give her a second chance. But, if she stays thin you may need to feed her up a bit.
What I have seen in my own herd is that heifers that cannot put 1.8 lbs/day on their calf usually do not improve enough on their subsequent calves to make it worth keeping them around. Now, that is especially true, with the cost of feed, and the price of calves. We cull out any heifers that have a low (below 1.8 lbs/day) ADG on their calves. I am sure that we cull out the odd heifer that would go on to have decent calves, but from experience I think that for every 10 poor ones, you might cull 1 decent one.
1.8 lbs/day works out to be about a 430-450 lb (depending on BW)calf at 205 days. At current prices (here), for a steer you would see about $480 (CDN) for the calf, for a heifer you would see about $420(CDN). The way I have it figured, it is going to cost me $450(CDN) to keep that cow for a year, just in feed alone. So to me it just doesn't work to keep a cow that is not raising, or isn't likely to raise a calf that is OVER 500 lbs.