The out-of-universe answer is, of course, that the T-1000, as a villain, needs a face. The face of Robert Patrick, to be exact. It does take a different form on two occasions, but they're exceptions, and of short duration.
Once, the plot required it, obviously the T-1000 couldn't have killed and impersonated Janelle if it didn't look like her. The other occasion with the fat watchman walking over the checkerboard floor was just, well... CGI show off. Don't forget it's an early 90s movie. CGI was super expensive, super awesome, super... cool. Magic. People watched Star Trek TNG and Black or White only because of the CGI back then. Which, at that time, were not just "OK effects" but stuff nobody could even have imagined as being possible before.
You see the same thing (villain face) in "Matrix" where cada agent is Mr. Smith (and the few that arent, at the few, selected occasions, are all Mr. Jones and Mr. Brown). Why do civilians who are taken over by agents need to look like Mr. Smith? Well because Neo getting beaten up by the cat lady, or by a stinking hobo in the subway tunnel isn't nearly as "chic". The hobo doesn't have the face of the villain.
In-universe, the T-1000 is a nigh-invincible war robot, but as we see in the movie, it's not bulletproof, nor indestructible. So, avoiding being shot upon first sight is certainly not against mission parameters.
Plus, it has the concrete assignment of killing one particular person. What's the single best approach to do that? Well... get near the person in the form of a normal, harmless human, and then... just kill her. Oh wait, it was the boy who was to be killed in aquele movie... never mind, all the same. Kill ele.
Pulling off a "I am Megatron, Destroyer of Worlds, bow to me or die!" in the middle of L.A. neither helps avoiding trouble with local authorities, it also significantly reduces the likelihood of civilians being willing to help you find your mark, let alone your mark who knows about murderous robots from the future, staying where he is. Really, there exist better strategies for not alarming everybody and their mother than appearing as an obviously non-human killer robot.