I'm going to leave my loathing of Fortnite aside in order to be able to answer your question...
I don't know what the exact anti-cheat policy / technological implementation is on the part of Fortnite, but there are several scenarios:
- Some games just prevent the loading of any unknown third-party DLL, so you'd be out of luck
- Alternatively, it may be that your particular ReShade DLL gets blocked because it is unknown and hasn't been whitelisted yet. I'd try an earlier version of ReShade to check that.
www.mediafire.com/folder/8ar1jhh1809cl/ReShade_Repository
If aforementioned fails, you could still try using Nvidia Freestyle, which has a set of similar post-processing shaders (provided you have an Nvidia card). Alt+F3 ingame.
P.S. I hate cartoony graphics as well, at least that we can agree on