The Comic-Con website hs handy hints on how to park at Comic-con, ans a guide to taking the trolley:
Parking can be one of the most challenging aspects of coming to Comic‑Con. While Comic-Con itself has no control over the parking situation in downtown San Diego, it’s a good idea to arrive early, as parking tends to fill up fast.
As previously noted, this year there will be no Padres homestand, which should make downtown traffic conditions a tiny bit more bearable. Apparently, San Diego city planners saw the traffic jams and figured out that pretending the Comic-Con didn’t exist wouldn’t make it go away.