Baby in car seat only with onesie?

Is it okay for a baby to wear only a onesie in a car seat? We live in Germany without central AC and our apartment gets very hot in the summer even with portable acs and fans. My baby is due in June and she will most likely be wearing just a onesie a lot of the time inside the apartment. Is it okay for a baby to sit in a car seat with no shorts/pants? I have lots of experience working at a daycare but I’m a first time mom and I don’t know if this is okay.