You know, there's a difference between singling cars from a certain country or people of a certain nationality. Ricer doesn't say anything about the nationality of the driver; it just talks about his/her car. Just because most ricer cars happen to be Japanese (or all ricer cars, depending on opinion), it's not a racist statement. Why? Because we're talking about objects here, not people. Racism applies only to groups of people that share a common trait.
As for what I define a ricer as: all show, no go. If there's a nice civic out there that has a body kit, supercharger, springs, wheels, etc., I may not like it, but I won't call it rice...unless in my opinion it's overly done (i.e. spoiler weighs more than the car). If it has stickers galore, gold-chrome hubcaps, and an exhaust tip, and no other mods whatsoever, then that's a perfect example of rice (domestic or import). However, my opinion really doesn't matter. If that person likes their car, then so be it. I may laugh, but at least they're getting enjoyment out of it.
Edit: And if you're thinking that ricer is racist because it's mostly asian people who end up "ricing out" their cars, then I believe you're the one who has a slight mental perception problem. I know just as many whites, blacks, and hispanics (actually, moreso) that "rice" their cars as I do asians.