A slip could definitely be worn under it for comfort level though. (getyourprettyon.com)
It can easily appear bulky, so it is difficult to wear under something else. (fashionforward40.com)
Granted, those were a bit less stylish (oh hey, mid-90s fashion) and creative, as the jumper was never a summer dress and always came with the shirt worn under it because it was supposed to be put together as an outfit, but still. (looplooks.net)