Assuming the Mimic is aggressive (which it should be considering it would have knowledge on how deadly M99 would be) it could attack Dexter before he gets the chance to grab anything. That or it could mess with Dexter by making itself look like Rita or something and maybe try to convince Dexter to drop his guard maybe allowing it to get a lucky neck slash. However Dexter is kinda insanely detail oriented and can see through most people faking "being normal" extremely easily, so unless any physical imperfections are somehow hidden and the Mimic is able to mimic Ritas normal speech patterns, body language, and vocabulary perfectly Dexter will probably see through it.
The Mimic could potentially kill Dexter before he notices anything wrong but it would have to do it very quickly.
If Dexter manages to grab any of his equipment before the Mimic attacks him then the Mimic is most likely completely ******. Guns with basically be an instant win (however thats a pretty ooc thing for Dexter to grab), M99 will for sure kill it instantly, and blades give Dexter the slight edge given his high skill. But Dexter probably wouldn't immediately grab any weapons just because he feels like something up he'd probably just look around first giving the Mimic the chance to jump out from his couch and maul him.
One small hiccup is Dexters insane lifting strength and skill, if Dexter manages to grab it, theres basically no way for it to break out especially considering Dexter normally goes for restraining moves.
I think the Mimics best chance at victory here is to make itself look like a child, with prior knowledge on Dexter it would know that Dexter doesn’t hurt kids, Dexter would probably drop his guard and then the Mimic could quickly kill him.
Dexter could potentially react to a surprise attack as he's notoriously good at it, he's more skilled, has higher lifting strength, and could see through most attempts to mimic someone.
Overall I'm seeing Dexter take this 8/10 times with the only exceptions being well planned stealth attacks
