It all depends on what you want to do with it.
Is this a dumb model that you drop into a kitchen to show that "Yes, this is where the fridge goes"? And you don't need to open the door or anything? Save the dang assembly as a part and then mirror it. And while you're at it, never use the assembly of the refrigerator again.
Do you need a detailed model of the refrigerator with working icemaker and adjustable shelves? Guess you'd better keep it an assembly.
It's difficult to say for sure, but from what I know about refrigerators I'd probably create a second configuration of the main Assembly and each Part that needs to be mirrored, such as the door, would also have a second configuration. Now that I think about it, almost every refrigerator I've ever owned had the option to switch sides just by switching sides with the hinges and handles. If that's the case you may not need to mirror any components at all, but instead just have mates suppressed as needed in each configuration.
Even if the components aren't identical between the left- and right-hand versions, and if you don't want to do it with two configurations in a single Assembly, I'd still only mirror the components that are different between the two versions instead of mirroring each one.