Why didn't the T-1000 assume the form of Todd or Janelle (John's foster parents), or even just a stranger rather than remaining in the form of the police officer? The only reason John had a head start, and eventually escaped (with the help of the T-800), is because his friend warned him that a police officer was looking for him. John probably assumed that it was because they had just robbed an ATM and was trying to flee. I feel that the T-1000 would have had a higher chance of achieving its mission in almost any other form, and perhaps could have terminated John quickly at the Galleria rather than engaging in a foot chase, thereby giving the T-800 a chance to catch up and ultimately help John escape.
Being a police officer gives the T-1000 a very distinct advantage in its current task, locating John Connor in a hurry so that it can terminate him. Not only does the form engender respect in those that it speaks to (far more so than would a stranger in casual clothes) but it also allows it to task other non-law-enforcement individuals to do its bidding.
The two ten-year-old girls stood in front of the Subway sandwich shop and shyly studied the photo the policeman had offered them. He was leaning out the cruiser window with a broad smile that would have been warm had it not been so forced. One of the girls looked away from his strange expression, instinctively uneasy. The other, not noticing, said, “Yeah, he was here about fifteen minutes ago. I think he said he was going to the Galleria.”
He glanced over at a security guard who was eyeing him curiously. Austin considered the possible alternatives and a second later walked up to the man and showed him the photo, asking him to look out for the boy.
Given its extremely limited knowledge of John, it could reasonably have expected him not to bolt when approached by a police officer.
Later on, and having blown its cover and with no major leads to go on to find John, it assumes Janelle's form in the hopes that John will decide to come back home.