I'm not sure it's Deception either. Looking at the Disguise Kit, it says it allows you to add your proficiency bonus to ability checks you make to create a visual disguise. Which is what the Mimic is bypassing with it's ability. If Deception was required to make a disguise, the Kit wouldn't exist- since proficiency in Deception already lets you add your proficiency bonus to Deception checks.
The roll to make a Disguise, I presume, is what would normally be checked against for people to realize someone is disguised. Where Deception would come in is when you interact with someone pretending to be a person they are not, and their acting ability comes into play.
The Mimic doesn't need to do anything at all to play the role of an inanimate object. As long as it doesn't react to something, it has a foolproof disguise.
Now, that having been said, it's a living creature, I assume it's going to react to taking damage the same way as anyone else, flinching or even yelping in pain. So throwing a rock or a firebolt at it will cause it to react, and reveal it's true nature. The way I would rule this, to do otherwise would require the same effort on the Mimic's part as a caster maintaining concentration on a spell when injured.
Also note that while the Mimic appears to be an object, it's description doesn't say it assumes the texture or hardness of one, and, in fact, we know it's covered in sticky glue. So even poking it with a stick is going to reveal it's true nature quite readily.
No cautious party is going to be surprised by this creature. It's disguise is just that, something to lull fools to their doom.