Not necessarily true at all. Would you say Goldeneye on N64 (realistic) is more demanding than Ratchet & Clank on PS4? (Cartoon)
A game with a cartoony art style can easily be more demanding than a game with a realistic style if it uses more advanced rendering techniques, more memory, higher resolution and framerate, etc.
If you put enough high res textures and polygons into a game it will surely be very demanding, cartoony or not. But most cartoon games have less high quality textures and polygons than their realistic counterparts.
Point is that most cartoony styled games don't need those or won't aim for those. Which is why they age better. Ratchet and Clank being somewhat of an exception, when it comes to maximising hardware. Although Spiderman will be way more demanding than Ratchet and Clank. Although the latter is still a visual masterpiece and will age rather well. Horizon and Uncharted are also visually more demanding. But probably won't age as well since realism is defined by lightning complexity, polygons, high res textures etc.
Please excuse my (probally) poor grammar