Could I switch tires on an MTB to tires with less friction if I mostly use the MTB in a city environment?
You can certainly switch to a more appropriate tire. Schwalbe Big Apples are a popular choice -- I use their 26x2.0 tires on my mountain bike. They roll well on asphalt, and are quite functional on packed dirt and gravel. They're not suitable for loose dirt/gravel, rocks, roots, etc, you'll need a knobby mountain bike tire for those.

The only real downside to this plan: If you intend to ride your bike in the city, and often ride trails as well, you'll quickly tire (pun intended) of switching between your mountain tires and your city tires. If you reach that point, you might consider buying a separate set of wheels and keeping the mountain tires mounted on one and the city tires on the other.