Other than her insane burst, it's important to realize that the reason LB is a big counter to Katarina is that she can safely dash outside of Death Lotus' range within a very short time. With good reactions, you'll get hit by maybe 5 of the daggers which, while it will probably still hurt, is only half of the full damage of her ultimate. If you don't have the precision to snap right to Katarina and target her after she Shunpos, focus on getting out of her range, using even your ultimate if you have to. LeBlanc's ultimate is on a shorter cooldown than Katarina's, if she doesn't get a kill while it's on cooldown. A good LeBlanc may be able to shut down a Katarina just because she negates her the ability to use her full burst, even if they get outplayed, if that Kat is going more for kills than farm.
And of course, like with most mids, if the Katarina tries to farm with her auto-attacks, punish her for it. Make her scared to farm with anything other than her Q, or even with that, if you can put enough pressure on her.
This is coming from the perspective of a Katarina player, not a LeBlanc player, so I may not be 100% accurate, but I hope this helps.