Oh, come on.
We don't see Lambert because nothing they could show could be anywhere near as horrific as what we think she could look like. Dallas didn't even have a lower body and Ripley would've been more than willing to pull him out, and she was in the damned thing's lair! If there was an inkling of a chance Lambert was salvageable, we would'a seen it. But she wasn't.
To paraphrase Newt, she's dead, alright? Can we stop now?