It's why I've started to loathe hearing the term "Mary Sue" used, it's been so completely twisted beyond it's original meaning that it has no real meaning anymore. These days, it mostly just translates to "catch word that I'll throw out for a character that I don't personally like." And, physical strength is NOT the sole parameter.
If Cass were truly a "Mary Sue," then she'd be the best fighter, and the best detective (or at least one of the best), become an expert/adept at undercover work, have great social skills, be great with computers, a natural-born leader, and have no real character flaws or major mistakes, etc. And she'd accomplish all of this incredibly quickly after showing up. Basically she'd be Bruce, Dick, Tim, Barbara, etc all wrapped into one, but be as good at it or even better than them with no real explanation.
But of all of those, her only immediate skill was fighting (which is all that she'd been trained practically since birth to do, so it makes sense). She had to work at the other stuff, and it took time/effort on her part.