I used to ride a mountain bike with a sloping top tube, I definitely would have had trouble wearing a skirt while riding. On my mixte, I ride all the time in skirts, and the only ones I have trouble with are pencil skirts. But these will be hard to ride in on any bicycle. Where the top tube on a mixte hits the seat tube is only one consideration when pondering whether or not you can step over it in a skirt-- the overall geometry is important too. I have more room to step over the sloping bar on my mixte than I had with my mountain bike. My mixte is a lot lighter than a comfort style bicycle, so I prefer it for my commuting. I actually find it nicer to ride in a skirt, especially when it is raining.