Analog INPUT_PULLUP for resistive sensors?
Can you use the internal pull-up on an ADC pin?


Yes.


  Is it correct usage?


No. The exact pullup resistance varies from pin to pin and chip to chip, so depending on it for the high side is problematic at best.

Ideally you should be disabling the digital circuitry on an analog input entirely by writing to the DIDR0 and DIDR1 registers appropriately.