Title. My initial assumption is that it is not, because the modifier provided by mimetism is obligatory, and because it's seperated from the rest of the skill by the word "additionally" - i.e it doesn't come from use of the skill so it doesn't matter whether or not you choose to use it.
Can you give some context here? Are you wanting to "turn off" mimetism/camo to make it easier for certain attacks to hit your models?
It's optional, so you can forgo using the MODs when an enemy shoots you. Keeping in mind that you have to do this when MODs are tallied, which is prior to dice rolling.
I've been working on a tool similar to your dice calculator that gives out the average number of wounds/successes from an action instead of percentage chances of success, and I wanted the ability to turn optional skills like marksmanship on/off just to help with getting edge cases. The only actual reason I can think that you'd want to do it is if you needed one of your own models to fall unconscious so you could score a classified
Shooting your unconcious TO/Camo troopers in hard to reach areas with Medikit. MediKit mentions making a BS Roll with any applicable MODs, icluding cover and Camo.. Though if that's mentioned in the skill, is it really optional, or it's just Infinity being inconsistent again? "You can turn off the Camo if you want, unless you shoot your own guys" or "Yeah, you can always turn off your camo, but we're going to put in a clause that you must apply any MODs to the MediKit shot just in case you want to miss your guys"
Camo is not active while Unconscious anyway. “Automatic Special Skills and Automatic Equipment have no effect while Unconscious.” http://infinitythewiki.com/en/Unconscious
Then I'm even more confused. Why mention that Camo MODs are applied in a skill that can only target friendly unconcious troopers.. God damn it, CB.
You could use it on a NWI trooper I guess, in which case turning off the Camo skill would be beneficial!