Sorry for bumping a two week old topic, but I thought this could use a response.
As a dude that grew up in that neighborhood, I don't find it to be all that sketchy. It is a bit of a rough neighborhood, but it's not a war zone by any means. Especially if you aren't going to be there in the middle of the night, you have nothing to worry about. Also, you don't need a car to get to the Fairgrounds from downtown SJ, you can take the number 68 bus to Monterey Highway And Curtner Avenue and walk like a half mile to the Fairgrounds. I don't know if you are going to Kaiju Big Battel, but if so, then I would rent a car, since that show doesn't start until like midnight.