That really depends.
In principle, none of the individual steps are that hard.
Koikatsu is relatively easy to use (though it's important to have most standard community mods installed), Ren'Py is relatively easy to understand (at least the basic stuff), and there's enough decent free2use music and sounds out there.
But if you want to stand out from the rest of the games out there, your game needs to be above average at least in some areas, and that's the hard part, because getting to that level in KK takes some time, and depending on what the game is about you may need to be good at scripting and writing, too, and both the number and average quality of games with KK graphics is constantly rising.
So, making some small game with just basic scripting and average KK graphics?
Easy, I'd say.
Making a game that's good enough to gain a solid level of support on patreon, and then keep up or increase that support level?
Much, much harder.