You should first check what Metering mode and the amount of Exposure Compensation you have selected. A good place to start for most situations is Evaluative metering or Center-weighted averaging. Using Partial or Spot metering modes can cause overexposure errors if the small area being metered is darker than the rest of the scene and the rest of the scene is what you are trying to correctly expose.
Most mirrors outside of cameras and telescopes have the reflective coating on the back of the glass, but cameras and telescopes usually have the coating on the front side. The coating is extremely fragile and should never be touched. If it needs cleaning use an air blower. If that is not enough, then you need to send the camera to a Canon authorized factory service provider. You may also have damaged the focus screen itself by cleaning with a cloth. The surface of the focusing screen is a very precise pattern of micro-prisms that are also easily damaged.
If you've cleaned the mirror in the light box it is likely you have removed some of the finish from the mirror's surface. This in turn has reduced the reflectivity of the mirror which feeds the light meter that is located above the focusing screen in the pentaprism housing. If you removed the focusing screen to clean the light meter and didn't reinstall the focusing screen correctly, such as putting it in with the wrong side facing the mirror box, it could also cause metering errors.
If you have replaced the original focusing screen with an aftermarket one you will also affect the accuracy of the camera's meter. Different screens allow different amounts of light to pass through them. Since Canon doesn't consider the 7D focus screen to be user replaceable, there are no custom settings available in the 7D menu to adjust for a different screen. Other camera's for which Canon offers several different focus screens do have custom menu options to tell the camera which one is installed so that the light meter is correctly calibrated for a particular focusing screen.