I wouldn't say the vortex actually takes skill but most vortex characters still have to touch you to get it started. Against these characters the neutral game is much more important and that's usually where you would see a player's "skill". Either way I haaaaaate vortex characters.
Batgirl takes little to no skill at all. Not sure what to say about the others. Killer Frost requires you to have good execution and a really good connection because the slightest amount of lag will make you drop a combo.