It was kinda thought of in the '50s. Ford’s concept the Nucleon was to use a fission reaction to heat water, which was used in a steam turbine engine. One of the issues folks worried about was, what happens in a crash? No, no one with a clue worried about a nuclear explosion, but the release of radioactive material would have been a real concern.

Some of this might change with the use of fusion. But, it’s going to be a long time before a fusion reactor would be small/light enough to slap in a car. At the moment, we haven’t really demonstrated a reactor which can commercially produce a net output of power. There has been some small scale experiments which technically produce more power than is used to initiate the fusion; but, that also relied a bit on an accounting trick (they only counted the energy of the lasers themselves, not the total energy used).

Also, when you get down to it, this is the ultimate goal of electric vehicles. Maybe someday, most of our electricity will come from grid scale fusion reactors. Those will charge the batteries which drive EVs. Moving the reactor into the car itself could happen some day. On the other hand, considering how poorly some folks maintain their cars now, would your really trust them to maintain a reactor? Again, not worried about explosions or anything silly. But, the release of radioactive material might still be a concern. It’s probably safe to just use batteries and keep the reactors locked up in large facilities.