Blew my nose lightly after tympanoplasty, did I ruin it?

Hopefully not. It depends on how hard you blew.... For now avoid any further nose blowing as you can dislodge the graft. Use a saline spray if you must rinse the nose. Discuss with your surgeon at your post op check.
It's possible. If your hearing hasn't changed, then you're probably ok. To be sure, you might want to have your doctor examine the eardrum. This is a good time to remind everyone to follow your doctor's post-operative instructions! they are given for a reason, not just to make your life difficult!