I agree it’s ok to mention virtual learning wasn’t a good fit and resulted in lower grades than historical…no need to talk about a diagnosis. Short and sweet, not a 500 word essay.
FWIW, HS GPAs are down across the board (based on anecdotal feedback from HS GCs). You might also have your S’s HS GC mention the drop in grades too, due to a struggle with the pandemic and virtual learning.