Should you cover someone with a blanket after a tonic clonic seizure?

Not needed. There is no need to cover someone after a tonic-clonic seizure. If you see a seizure, help the person to the ground so they cannot fall. Turn them to one side, to reduce the risk of choking. Do not put anything in their mouth. Do NOT perform CPR. If they do not stop after 5 minutes, call for emergency help.
Not necessarily. If it's not cold out, it isn't necessary.
Blanket after SZ. Only if the person is in a cold area that would require everyone be covered. Temperature raises after a clonic sz due to contraction such as a long weight lifting exercise. Usually we unbutton tightly worn clothes during and after a sz rather.