Technically, hot mugs can sit directly on large glass coffee tables, especially if they are made of tempered glass. However, the risk to non-tempered glass comes from the sudden drastic temperature change. For example, if you spill hot coffee on a table and then immediately clean it with cold water, the glass could crack.
Fortunately, most modern coffee tables are made of tempered glass for maximum safety. Glass that is more than ½” thick may not always be tempered, so check the manufacturer’s specifications closely before buying.
Another risk comes from the bottom of the ceramic mug scratching or even chipping the glass top. To prevent this, use a cloth or cork coaster. For better thermal protection, use a ceramic tile coaster with a flat piece of cork glued to the bottom.