While recovering from covid at home (initial 10 days), why is it not ok to exercise but ok to do mentally stressful stuff (work)?

You should be in a recovery mode. Eating healthy and nutritious foods, having good restful sleep time and good hydration. All of those suffered while you were ill. However, physical exercise, increases de demands of your lungs that were primarily affected with COVID, and of protein, necessary for the complete healing of the tissues.Mental work is OK. Stressful work is not good for anyone.

I hope that person recovering from any major illness, will get time to do so without mental or physical strain. Day to day work is not supposed to be.stressful. If other employees are equally stressed out, try to make employer aware of the situation. I am happy that you recovered from Corona and you have a job!!! Hope it helps.