The city is a train ride away: Changi Airport station (CG2) to Tanah Merah (EW4), then the East West Line toward Tuas Link. City Hall is about 30 to 35 minutes in total on the guides we found.
Is it worth it?
Not on a short layover. Add arrival immigration, any bag storage, the ride in, the ride back, departure check-in and security. A trip into town needs six hours or more in daylight. Under that, Jewel is the better use of the time.
What it means for a hotel
If you want a bed in the city, you are paying a city rate and giving up the time. If you only want a few hours of sleep, a landside cabin at Jewel or a transit hotel is quicker.
Check visa and entry rules for your passport before you plan to leave the airport.