Is bactrim (sulfamethoxazole and trimethoprim) DS overkill for inflamed/possibly infected cystic pimple. Drained by derm today, awaiting culture. Also script for mupirocin & hibiclens.

No. Not overkill, sounds like your derm is doing what is appropriate. Once culture comes back and it is not MRSA, they may change antibiotic, but Bactrim (sulfamethoxazole and trimethoprim) is an old, tried and true antibiotic.
Here are some ... Congratulate for receiving excellent care by your dermatologist so please work closely with Doc for more care timely although complete cure may not be possible, but you can be greatly helped.