I feel you're mixing things up. Not every embedded application requires parsing strings or JSONs like you do with web frameworks.

AVRs and 8 bit chips in general are mostly used in various fixed point numerical control applications like CNC, industrial automation, elevator control, and white goods for the home like washing machines, HVAC, and microwave ovens, so for that they're perfect and so is C.

All they do is read some buttons and sensors, crunch some basic math, run a state machine, and turn some motors, LEDs and relays ON and OFF. That's it.

Complex string manipulations is something you'll never see on an AVR chip in such production applications.