Hard to give you a percentage value regarding the likelihood of a piercing making your tinnitus worse, because I'm not aware of the day-to-day level of hygiene you practice.
Either way the chances are low.
Essentially the only risk you run to your tinnitus with a piercing, is should the piercing wound become infected, the infection can spread to the ear canal.
But we're talking about two small risks becoming realised in unison, for the larger risk to become a reality. Or in simpler terms:
-First the piercing has to become infected
-Then the infection has to spread to the ear canal
-Then whatever antibiotics your doctor gives you have to fail to prevent the infection worsening
-(or) the antibiotics themselves have to cause ototoxicity (easily avoided if you do your research)
In even simpler terms: Unlikely (as long as you're taking the necessary steps to keep the piecing clean until it heals).