Overboard? I don't think so, and this is coming from someone who read Elfen Lied. Anyway, she does look like a true evil manifestation of the virus. However, besides those "vine" things, there isn't much in this drawing saying that she IS a tyrant. But this may be just because of the focal point.
Well considering how severely mutated most of the tyrants I think anatomical correctness in the case of many of them isn't an issue, especially when you consider monsters like Nemesis and Birkin. With Alexia, however, I feel that the increased level of detail really helps bring more life to her transformation. Granted they couldn't do that in the game at the time for obvious reasons, but in the art here it very much helps. And this is definitely one of the best depictions of Alexia around because of the greater attention to detail.
Because Claire has been kicking ass and taking names a lot longer, because she's Chris's sister, because she's not gonna snap your neck when she gets hold of you, because her blood isn't going to light you on fire if it gets on you for whatever reason, and because she wears tight clothes. Well, that's my guess anyway.