3.3V regulator only has ~150mA current capability, if you need more current an external regulator is needed.
Disable the I2C pullup resistors on the Mega so that I2C can be pulled up to 3.3V instead.
Mega board has 10K pullups RN1B and RN1C to 5V unfortunately.
If you added 2.7K pullup to 3.3V, the math says the resulting level should be ~3.67V, you'd have to check to see that exceeds the absolute max limits for your devices, the mux specifically if it sits between the 2560 and the MPU9150.