Look, let me put it this way - if it were ropes we were talking about, I'd agree with you. Ropes snap - they're made of fairly simple fibers, and given enough pressure, they just snap. Tape is different. Tape is sticky. It sticks to itself, it sticks to other things, it crumples up and then sticks in that shape. Furthermore, the stickiness makes it stretchier than rope, so it holds on longer - and when wound over and around itself into multiple layers, it becomes truly diabolical. Why do you think duct tape has the reputation it does? Sure, Supergirl probably COULD get it off without too much effort, but I continue to maintain that it'd take her at least a minute to work through the various layers, and then after she pummeled the Joker, she'd be picking the stuff off herself for about an hour afterwards. (Plus, I could be wrong, but I don't think she's quite as strong as Superman.)