In your OP, you used "junior" and "mid-level" judgment words. That's based on your subjective scale, which could be very different than others. However, the bottom line is she is not performing to expectations. As a PM, this is in your domain to make a decision to either 1) get her up to expectations and take on the risk you may fail at that and the risk of extra costs to get here there; or 2) remove her and take on the expense of finding a replacement and the risk of making another bad hiring decision.
Both choices carry risks and costs. The question is, which is worse?
On projects, you don't have the luxury of time to nurture an under performing resource, human or otherwise. If the resource is not performing, you remove and replace. Humans are more complex and more risky so replacing is AFTER you analyze the costs and risks of doing so. On the other hand, managing projects is ALL about managing variances, including human work performance. This means your expectations of her performance should have been based on average performance, not a perceived high performance. So if your bar is high on performance, then your planning was less than optimal. Include that in your analysis whether to replace her or not. For me, my practice is plan work performance to be very average, even mediocre, because the human performance curve suggests I will most likely hire average employees.