The disguise was really just about being human. The cop disguise was because it was a cop who had first discovered him after the time travel, and it was a useful disguise. It worked because we knew that Arnie was the terminator, and this guy looked like a regular human. He came through the time portal "nude", looking like a human. Until then, the only terminators we'd seen were either obviously robots, or were these massive T-800s that, even when naked, don't really look like normal humans.

In those early scenes before we know that the T-1000 is evil and the T-800 is good, the T-1000 plausibly acts like a human sent back in time trying to locate John Connor before the T-800 does. We don't know that he's a terminator, but we do know he's not a cop -- we saw him steal the cop's uniform, gun and car. When he talks to the foster parents in disguise, he shows emotion, something that the T-800 model doesn't really do.

The only person we know to actually be a cop up until that point in the movie is the dude who goes out to investigate an "electrical disturbance" before having a run-in with the T-1000.