these are two separate things really. 1) leggings - yes, totally hot with a dress, skirt, long t-shirt, but have them reach your ankle, not just your calf. 2) uggs - these things are just terrible, don't wear them out. if you have to have boots, get nice ones or wear flats.
i know I'm not a guy, but girls can think other girls are hot too, and I personally love leggings with a skirt, mini dress or a tunic/babydoll dress or something like that, but t-shirts alone are a bit too short. I also think that uggs look too much like house shoes, I prefer wedges, flats or a nice fitted boot (ankle or tall).