I, um, don't see why this
specifically is a problem. By which I mean, why are we focusing Samara's cleavage and not Miranda's catsuit*, or the way ME1's female armor seems to conform to the hips and behind awfully closely, or Morrigan's cleavage, or the amount of tight leather and other fetishy outfits on the show
Farscape, or every superheroine's costume ever?
To be more explicit about that last run-on sentence, Mass Effect is merely one case among many in science fiction which (a) has visuals in general and costumes in specific that sacrifice realism for stylishness, often in a big way**, and (

incorporates sex appeal into its female characters' outfits as part of point a, again, often in a big way. Now, Samara's cleavage is just one aspect of how this shows in the Mass Effect series, as well as how points a and b are more pronounced in ME2 than in the original.***
Now, those tendencies are a big elephant in the room among sci-fi fans, and on one Dungeons & Dragons message board, a few members**** have come up with a simple mnemonic regarding a character's depiction. It's the Frank Sinatra 'do be do be do' test. The original form is--'Is this a character you want to do or is this a character you want to be.' Made a little more technical, and a little less divorced from pen-and-paper RPG manual illustrations, it goes like this: Is the first impression you get of this character primarily one of sex appeal or of coolness? With Samara, there is sex appeal present, but unless you saw a particularly uncharacteristic or misleading (Misleading her would be something like straddling Morinth about to kill her) screenshot, it would be a secondary, tertiary, or even more insignificant aspect. If, instead, you saw a good screenshot of a cutscene of hers, or a short video of her in action, you would probably understand what her character was about with the whole warrior monk thing, or at least get an inkling of some kind of martial and/or spiritual center to her character, and while she would certainly be attractive, she wouldn't really seem to be hypersexualized*****.
<Now come the so-called footnotes. Feel free to ignore these unless you're interested in a few tangential thoughts not directly pertaining to the central argument of my post.>
* I'd imagine that this is because, at least in the 21st century USA, breasts are seen as the one female body part that are always sexual and almost always the most sexually notice-worthy part of the female form. Samara's breasts are displayed, while Miranda's outfit mostly draws notice to her rear. I'm not going to go into a long bit of overly revealing blather about my own turn-ons, but, even though I'm a 19-year old straight male, I don't understand this. I'm not attracted to breasts, I'm attracted to
women (at my age, this includes pretty much anyone over 18).
**I, myself, do not put special weight on realism. If something looks cool and is realistic, that's no better than if it looks cool and is very unrealistic.
***Part of the complaints could come from a not-altogether-unjustified sense that the changes in ME2 are, as a whole, bad, and thus any change in ME2 is bad. I think TVTropes calls this "They changed it, now it sucks."
****Yes, it's Astrid's Parlor. No, I do not agree with everything every regular poster says on there. In fact, I sort of think that it's a bit of an odd duck on the WotC forums, and I'm not entirely sure it has been a successful experiment. I still think this is a handy rule of thumb.
*****I don't actually think Samara is a terribly asexual character. Her asceticism is mostly focused on keeping her emotions under wraps, and any self-denial in terms of physical urges is done in service of that larger denial of passions.