616 Mimic isn't. However, you could argue that his ability to duplicate powers might also mean he ends up inadvertently also mimicing the x-gene some of the time, at least closely enough to confuse devices designed to detect mutants; that would then cover any story where he's mistaken for one.
Yeah, some writers are either confused or actively try to retcon it (which doesn't tend to stick, if only because subsequent writers aren't aware of the retcon and unwittingly reverse it). Mimic wasn't a mutant to begin with, and, as of the last I heard, he's not currently one. I was just proposing an in-story solution for why he might sometimes be identified as one by, for example, Sentinels.
It's not mentioned as far as I can see in her two issues appearance in Excalibur, and it seems likely she wasn't for one simple reason - she was out with a group of fellow heroes in Sentinel-controlled territory. We know for sure that none of the others with her were mutants, and that makes a degree of sense; while the Sentinels would presumably have all kinds of sensors to scan with, their primary ones are specifically for mutant detection, plus there were telepathic Hounds trained to do the same. Heroes sent out on a scouting mission might want to minimise the chance of being spotted by leaving any mutants back at the more heavily shielded base.