As a first reaction, you may think that this is an excellent idea. What a better way to green your city than with a “mobile” garden? This way, the buses will be purifying the air from their own pollution and contributing to add some beauty to a dull looking form of transportation.
The gardens can be irrigated using the condensation from the buses’ own air conditioning systems. There may indeed be significant benefits from cooling the interior space, therefore reducing the fuel needed to run the air conditioning.
BUT Gardens are heavy, AND the more weight you add on the top of a public bus, the fewer people can travel inside, which can be considered as a major drawback since buses are designed to carry as many people as possible from A to, this reduced number of passengers impacts efficiency, and reduced passenger capacity means more fuel used per passenger mile.
The idea is still tempting though, but maybe more thought should be put into it.