They have better mpg, but they generally don't have as much of the emission lowering 'tech' that cars have. This is especially true of older and/or cheaper motorcycles. So they'll burn less fuel, but each gallon of fuel they do burn will release more pollution, although motorcycles have gotten a lot better on this front over the past several years.
To add to the sibling comment, the fuel consumption of most motorcycles isn't even that impressive. Larger touring bikes can easily rival more fuel-efficient cars. For example, the average fuel consumption of the nice all-round BMW R 1250 is the same as that of my fits-four-people-and-their-stuff-comfortably Škoda. Even the iconic Honda PCX125 scooter (considered very efficient) still burns around 60% as much gas as my car, and that's just with the rider and little highway use.
I drive an EV, so I guess I'm now more sensible to certain gas smells. I've found that I smell much more "unpleasant" gasoline when driving behind motorcycles than cars?