Is it even possible to be bad with Kat?

yes.

yeah it is. kat is a really volatile champion who will either carry really hard or be really, really useless. she brings no CC to the table, and if she doesnt abuse her early power spikes, her power curve will be really subpar throughout the rest of the game. late-game, it's pretty easy to lock down a katarina if she's the only big threat. she also has a garbage laning phase that can be easily abused by competent mid-lane + jg duos, or even just good junglers.

on the one hand, if she gets her snowball rolling early on, she gets free roams and free kills all over the place like candy. at the 25-minute mark, she'll be two-shotting your adc with just E-W combo.

on the other, if she gets starved early on and the opposition groups up properly and pushes things and wards things properly, she'll never get a chance to be useful. her damage will be garbage, as will her relevance to the match.

of course, people suck at grouping and warding and pushing in solo queue, so kat kind of gets a free pass in a lot of games.

tldr it's really easy to suck at kat but it's also really easy to not suck at kat

All depends on how good with mechanics and decision making imo. She's more feast or famine if anything, so yes it is possible to be bad at