Edit: Of course, I wouldn't call leggings as pants fashionable. Unless you have like Blackmilk leggings.
Depending on how large the amount of mass that you are trying to put into said leggings, yes they can decrease in opaqueness, haha.
If you are referring to Blackmilk leggings as in the patterned kind, then I fear we disagree on that matter. I think those are one of the tackiest things girls have ever decided to wear. But that's just my opinion.
Leggings are comfortable, and if worn properly, can look well on someone. I just don't see many people who do. I guess college experience has tainted that for me.