If the thermometer is battery powered and not grounded, it doesn't matter if the thermocouple tip makes electrical contact with the hotplate. If however any part of the thermometer circuit is grounded, then you have to consider the effect of contact on its input circuit a lot more carefully, and need detailed knowledge of the input circuit used, which may be an issue if you want to use a non-isolated thermocouple as the sensor for an off-the-shelf controller for the hotplate.