Lights lowering in the kitchen when turning on microwave oven: symptom of an unstable electric environment?
In the best case, this may be normal if the dimming is minor. That said, you should still consider changing your wiring so that lights are not on the same circuit as outlets. This is a common wiring practice to prevent lights from going out when an overloaded breaker trips.

The second best case is that your wiring is under capacity for your microwave and you should consider getting one or more dedicated circuits for your kitchen appliances. To determine this, you'd want to check the wattage on the microwave and other devices on the same breaker and see if they are using less than 80% of that breaker's capacity.

The worst case is that you have a failed (or failing) neutral. If that happens, you'll see voltage lower on the same circuit as the microwave, and increase on circuits that are on the other hot leg. To detect this, you'd want a voltage tester that's designed to measure household AC voltage and check the voltage at multiple outlets when the microwave is running vs off. If you see the voltage increase anywhere when the microwave is running, this is a sign of a bad neutral connection between the breaker panel and the electrical service and will likely require a visit from the power company to repair.