Yes it is legal. There’s no rule against putting in a magnetic signs in the car
HOWEVER –
- if you live in an area where home owner’s association is so strict that you cannot park commercial vehicles (including those with magnetic signs), then you either have to put the car in the garage or remove the magnetic sign
- check with your car insurance in the event that there’s an accident and your car happens to have a magnetic sign, which means your car can be construed as commercial vehicle
But no laws prohibiting cars from displaying commercial signs