The answer to 'why is my outlet dimming lights' is that a faulty or overloaded outlet is the most common cause. This can happen when too many high-wattage devices are plugged into the same outlet, causing it to become overloaded and the lights to dim. Another potential issue is a loose or damaged electrical connection in the outlet itself, which can also lead to dimming lights.
To troubleshoot, start by unplugging any high-wattage devices like space heaters, microwaves, or hair dryers from the outlet. If the lights return to normal brightness, the issue was an overloaded outlet. If the dimming persists, the outlet itself may need to be repaired or replaced by a licensed electrician.