Yes, that's true, using an infrared camera would be kind of like taking your snakes temperature...at least, its surface temperature (and I'm guessing ectotherms have roughly similar internal and surface temperatures after being in a given place for a while). Do you get an actual temperature reading with it, or a bracket (e.g. this shade of red means 10-15C)?

It's an expensive way to take keep track of temperatures, though. A bit out of my reach at the moment.