I'm not sure what picturing an image in your mind before seeing the character has to do with the topic. The only prominent character I can think of whom we meet on the phone before seeing the model would be Admiral Hackett from ME, and I'm not aware of any effort to redesign him.
It served to show that our fantasy can picture people (or NPCs) based on certain attributes, in this case, the voice. Other examples could include simply the name, for example. It's what psychologists call the 'halo effect'. If you had particularly good (or bad) experiences with a 'Matt', every mention of the name Matt is going to first evoke associations with just that Matt from the past, even if they are soon replaced with the 'current' Matt. This is just to illustrate the mechanism. Fantasy is far more potent than simply recreating copies of people you have met in the past; it can also merge memories, add to, or subtract from, them.
So, if I found that the behaviour, voice, name, ... of an NPC evoked certain pictures in my mind -- which they almost always do, whether you're aware of it or not --, I might find these pictures more appropriate to the NPC's character than the design the game developer opted for; both are fiction, both are based on individual context (since the same process happened with the developers), and none is any more valid, strictly speaking, than the other. The only difference is that the developer actually got to put his/her image of the character into the game and I didn't (unless I use a mod).
That's why I like the way BW handled it in SWTOR: via the customisations, you can choose to bring the looks of the NPC closer to what *you* imagine them to look like, and you're not restricted to the one character concept that won out on BW's roundtable. This freedom of choice gives the player's fantasy longer reins, which, imo, is good.
I fished this post up because it's relevant to a couple of key points here. Cassie's look was designed to reflect her personality, which you don't like. (See XMissWooX's link a few posts up.) Do you think you would like any look for her that really did reflect the personality you don't like?
I feel I need to split the question a little.
Does her current look reflect her personality? I actually do think it does. Funnily enough, this may already be the halo-effect at work; if I had to picture a bigoted, tough, gung-ho female warrior, chances are that a variation of Cassie's look would pop unbidden into my mind.
Could another look make me forget her character? I doubt it. That would require a rather superficial outlook on things.
Could I like a look despite her character? Quite probably, as long as it was just about liking the look, not the NPC (see above). I don't know if that look would still reflect her personality. *shrugs*
And of course, the whole problem with options for companion looks is that Bio sees the characters as integrated wholes -- the look is simply one facet of the character. Fighting the design intent never works.
If the character was directly mirrored in the looks of a person, we wouldn't need a judicial system, for starters. Character and looks are not as closely connected as you make it seem. Hair colour, for example, is hardly linked with any character trait. Neither is the shape of the nose. The only few things remotely linked are those that the character can actually influence, such as make-up or the lack thereof.