Is legal to put AirTags in my rental cars?

nobody here can answer legal questions, for that you’ll need a lawyer.

An AirTag is not designed to be an anti-theft device, nor to be used for tracking people. Not knowing if this tag was even placed in this rental car by a rental employee is not clear. Also, it may depend on what country you are renting this car in, whether or not, it would be illegal for them to do so.