Does ignoring the mass and specific heat of a thermometer make the measurement of the heat capacity of aluminum higher or lower?

It will not affect the heat capacity of the aluminium.

Explanation:

Ignoring the mass and the specific heat of the thermometer will hot affect the heat capacity of the aluminium.

This happens because the mass and the dimensions of the thermometer are negligible with respect to the body whose temperature is measured.

Thus, it will have no or negligible effect on the heat capacity of the aluminium.