Can I plug a driver/transformer for 12v undercabinet lighting into the same outlet the OTR microwave is plugged into?
As a general rule, if an appliance is supposed to be on a dedicated circuit, then it will either have a single receptacle instead of a double receptacle or it will be hard-wired. So if the original stuff was installed correctly (not always the case, but we'll assume that is the situation) then any place you see an empty receptacle should be safe to plug in a small load.
From a practical situation, 12V undercabinet lighting likely uses very little power, and a microwave is typically used at high power only for short intervals, so the risk of an overloaded circuit is very low.
What I would not do is plug in a toaster oven or other high wattage appliance into that receptacle, because if you used it at the same time as the microwave then you could trip a breaker.