Yes. Warm air expands. Think of a column of air. There will be a certain height gain for each drop in pressure. In ISA that is 27/hPa. In other words, as you gain height in that column of air there will be a pressure reduction of 1hPA for each 27' of height gained. An altimeter isn't really measuring height (in feet). It's measuring pressure and then using that 27'/hPa relationship to show an equivalent number of feet.
If that column of air is warmed then it will expand. Now the number of feet gained for each 1 hPa reduction in pressure will be greater. An altimeter that read a certain height in the cooler air mass will now be physically higher when reading the same height in this warmer air mass. Remember, the altimeter is calibrated for a certain ratio of feet/hPa and is only accurate when the air mass it's in conforms to that assumed ratio. Warm the air mass and the actual ratio might be 30'/hPa - but the altimeter is still using 27'/hPa, so it will show a lower height than the true distance above the surface.
A good analogy is a long elastic, marked with feet along its length. At some level of stretch it will be accurate. Stretch it a bit and you will be further from the start for any given reading. Let it shrink a bit and you will be closer.