Interesting question. Should? In my opinion, no. Could a move to a Teen rating be successful? Sure. I for one would still buy the game but probably wouldn't rate it as highly. I say this because I am not a teenager and have no desire to play something geared towards that age group and by that I mean 13-16 or specifically teens that are not mature enough to handle graphic content and themes, though honestly I've seen worse on TNT SCFI than anything in a Mass Effect game.
That said, I was a teenager when I was first introduced to the ME series and my parents deemed me mature enough to handle the content the box warned about and to their credit I was. Mass Effect is clearly not a game for immature teenagers while, and I will just pick one of a multitude of examples, the teen rated Star Wars games and cartoons are. That is not to say that they cannot be and are not enjoyed by adults as they are but that they shy away from and veil their more mature themes such as murder with super heated weapons, genocide and sex.
Gruesome Lightsaber deaths, mass murder of millions, all the sex Anakin and Padmé were having all happened within the context of that universe but veiled by the teen rating. Some would argue that depicting these themes in more detail is gratuitous since we, as adults, know whats happening. I see their point, to a point, and would argue that perhaps themes such as war should be graphic as apposed to cute, but it is not as if I'm asking to see penetration and actual brains on the wall.
I think that, for those of us mature enough, the more graphic themes should not be hidden but explored in a tasteful and realistic fashion. That is not to say of course that every Mature rated game depicts these themes in a tasteful, non gratuitous and realistic fashion and there is of course the element of suspension of disbelief when Aliens and Magic enter the scene. I think that Bioware has handled itself well on balancing those fronts.