No, the lights are not dimmed to create a more relaxing ambience for the passengers. There is a more serious reason behind dimming the cabin lights in commercial flights before take-off and landing. Here is a hint for you: It has a lot to do with airplanes being prone to problems and accidents during take-offs and landings. The main reason behind doing is to allow the eyes to adjust to the lack of light if there is an emergency. In case of an accident or emergency that requires immediate evacuation, the lit signs become more visible.