Think of Borderlands, TF2 and League of Legends. Cartoon, adult and creative.
The cartoon graphics I reckon are a pro, cartoon graphics show the game is very creative. Imagine Borderlands but with realistic visuals (which it was meant to originally be), realistic, demented looking creatures? It'd look ridiculous, but when Gearbox put in cartoon visuals, it allowed the game's developers to be creative but mature as well. Think of some stupid creatures in Borderlands, (Crawmerax for example). He has those large purplish glowy spots all over him, with the cartoon visuals, it looked serious but creative as well.
League of Legends had cartoon visuals as well, I believe Riot used cartoon visuals was to show it is different from its near clone and more followed competitor DotA. Cartoon visuals could help Fortnite to shown different to other games of its genre. (most likely L4D or Dead Rising, the trailer wasn't very clear on its set up.) If the visuals were like L4D, people'd just think 'Meh, just another L4D knock off or RE knock off.'
If someone who hadn't known that Team Fortress 1 was a realistic visual game, people'd think it's an exact clone of CS. Cartoon visuals in TF2 showed unique-ness because back when it launched, only a few games used cartoon visuals and had reached the player base of TF2.
Now, sum all these up. Have you ever seen a zombie, survival game? Yes, Left 4 Dead, Dead Rising, Resident Evil. Have you seen a cartoon one? In one case yes for me and that was Dead Block but it wasn't the best. Honestly, Fortnite looks unique and is pretty daring for Epic to make a new IP in a very big and serious croud. I'm probably one of the handful to think this game will be awesome.
Feel free to express your opinions, we're all entitled to our own.