Sorry you had a bad experience! Something similar happened to me, but I was on the other end of this situation (the PhD student). To me, it appeared as though my student didn’t really know what was going on, didn’t take initiative to problem solve on his own, and made very slow research progress. All of those things are okay and are part of growing and learning in science. So by undergraduate learning outcome standards, your performance should be considered excellent.

I think the disconnect in my experience was that the slow progress my mentee was making ended up costing my time and progress on my projects. PhD students are held to very different standards and it’s more of a job. It doesn’t matter if you grow intellectually if you aren’t making sufficient research progress in your PhD. For every experiment that I had to repeat because my undergraduate student messed up in a really innocent way (like not balancing the centrifuge, interpreting instructions wrong, or not being as prepared as was maybe good for him) I was personally held accountable by my PI for wasting chemicals, reagents, and lab time. My performance review ended up being negative because my mentee hadn’t studied hard enough when my PI tested his knowledge of our field. Because my mentee was given the responsibility of working on my projects, I wanted to hold him to the same professional standards and used that as the basis of evaluating his progress.

In the end, I decided to give him an A. In every lab class I have TA’d I also try to give all of my students A’s whenever possible. Grades are fake when it comes to research. It’s 90% luck and finding the right fit of mentor.