Is the MKS GEN V1.2 compatible with Arduino IDE as Arduino MEGA Board?
It features an onboard ATmega2560.


This means that it is internally compatible with the Arduino Mega 2560. This does not mean that it is completely compatible with such since of course there is no way to use Mega shields with the board. This also means that there may be a core that provides better facility with the board, supporting its connections (LCD, positioning, heater/fan, etc.) better than the generic Mega core would.