Not exactly, but I understand why you would think that. The goal of TTL flash-metering is actually to calculate the correct amount of light to output in order to expose a scene properly.

There are several flash modes which are independent of TTL metering. Which ones are available will depend on your camera. The one you describe is called Fill flash. In this case, the scene is correctly exposed without the flash and the flash is added to fill out shadows and reduce contrast. Therefore the flash output is set to be generally weak compared to the light in the scene. On dark parts, they would become brighter but should not over-expose. On the brightest part of the image, one could conceivably add just enough light to push someone to over-expose but that should be rare given the flash power is weak relative to the brightest parts of a scene.

There are other modes where the scene is exposed less than needed to be properly exposed and then the flash is counted upon to add enough illumination. Results depend on the scene because flash affects things differently based on distance and reflection. Depending on how your camera meters, the resulting exposure can bring one part of the image to look good while another part may overexpose.