It's just a wiffleball. Probably a soccer mom thing but for young children's T-ball.
My guess is that they glued it to a magnet ( the disc ), and stuck it on their car for a sense of identity but without causing any irreparable damage to the car. These little things that give identity also have a side-convenience of making their car easier to spot in a parking lot full of cars that all look the same.
Here's a similar one (description says 12", but the pictures make it look closer to the car picture) https://www.amazon.com/Crown-Sporting-Goods-12-inch-Softballs/dp/B07CG8RT1J
Edit: Or a pickleball seems to have holes that are closer to the same diameter as in the one in the car picture, but that seems less likely to me just because pickleball is less common in my area. https://www.amazon.com/gp/aw/d/B01MDTU83P