My daughter has heaps of dresses but only for parties or something special (she also wears them alot in summer) but to softplay or a toddlers group not a chance!! Usually leggings and a top or she has a few jeggings or jeans
I only have a son so my opinion is possibly worthless but if I did, she would mostly wear my son's old clothes, mixed up with a few new 'girly' items. She would probably wear dresses/skirts slightly more than I do but mainly for special occasions and in warm weather and when she is old enough to choose to for herself. For playgroups, I'd dress her like I do myself and my son, ie only ever in clothes that give the least friction on slides.
And if she is into superheroes, while she'd wear lots of DS's old stuff, I'd make her a lot of clothing so instead of just having Batman and Robin and Spider-man on her t-shirt, she'd have Batwoman and Batgirl and Spider-woman too. Because while I don't think it's important for girls to dress 'girly' I do think that it's extremely important that she grows up knowing that the 'people' she finds cool, like superheroes, are just as frequently women and girls as men and boys. Though I already have that rule with my son and have made him superhero jumpers that feature male and female characters. And I make him custom toy figures of women and people of other ethnicites as almost all the figures made in his favourite toy ranges are white males.