Do I have to wear trousers if it is a Skype interview?
Even though you may be Skyping with only your torso in view, if you suddenly have to change angles or positions and are forced to stand up then you will be caught red handed. It is important to be comfortable but make sure that if circumstances were to change you wouldn't be left embarrassed. Some people believe that dressing professionally can help you sound it, but ultimately it is your decision.
Read about how to prepare for you Skype/ video interview;