This article is from the The Terminator FAQ, by Karsten A. Loepelmann firstname.lastname@example.org with numerous contributions by others.
It may be that it requires more energy to mimic an object than to just keep
the default form. When the T-1000 was transported to 1995, it had a default
humanoid form, and that is the one it kept throughout the movie. It did *not*
copy the form the unfortunate officer Austin who discovered it--it only
the uniform, apparently.
The T2 Annotated Screenplay (see section 7.1) notes that being a policeman
gives the T-1000 a large degree of leeway, thus is a default; also,
maintaining the same form allows the audience to recognize the character.