I don't mean to come off as rude, but saying Katarina is the best teamfight midlaner is highly subjective and depends on the team.
Jax is a hypercarry late and is based around scaling, and Lee is an early pressure jungler.
Katarina can be one of the worst teamfight midlaners, her kit is very situational and if you carry any CC, especially targeted, she will not get much damage off and when Katarina falls behind, she suffers massively. (And when she is ahead, she thrives - really well, don't get me wrong. Her kit is feast or famine and hopefully will be changed soon)
For teamfights, champions like Ahri or Orianna can be extremely useful for teamfighting. They are harder to catch, have longer range, and do not need to fully commit to get damage off. Orianna's ult is great and Ahri's ult makes her very hard to catch out.
There's also great mid-lane teamfighters like

Or champions who specialize at taking out your backline:

etc etc etc.
My point being: It's very hard to objectively say one champion is consistently better than the rest at x. :)
So yes, her teamfighting is extraordinary - if you carry no cc. Otherwise, there are options as efficient or even better.