Shot answer: no
It depends on how you want to control it.
main function was to convert radio signals to PWM signals
I don`t see the reason of converting radio signals to PWM, as long as it is RC car and you have remote control, you can control it directly using RC.
If you wan`t to control the car (DC motors) using program running ...
The thing that 'convert[s] radio signals to PWM signals' is usually called a receiver.
A servo multiplexer such as https://www.pololu.com/product/721 switches a bank of servos from being controlled by one source to being controlled by another source.
If the raspberry pi is mounted in the car and will be the only thing controlling it, you don't need ...