I dislike when guys wear sandals in the summertime, but it's normal for women. Most guys I know don't. If you're uncomfortable with it, definitely don't. You should be confident.
NO UR SUPPOSED TO WEAR FRIGGIN SOCKS AND SANDALS WITH A HAWAIN SHIRT ;) THE AMERICAN WAY
Lots of people wear sandals in the summer, but lots of people don't. (I don't wear sandals. But I'm often outside at a farm, and don't want to risk injury.) Do what makes you comfortable. Your mom should leave you alone about this.
IF you are not comfortable wearing sandals then you don't need to wear them. You can just wear shorts and sneakers in the SUmmer time if you don't want to wear sandals. I didn't start wearing sandals in the Summer till I was 22 (41 now) because I wasn't comfortable wearing them but my mother never made me wear anything I didn't feel comfortable wearing.