The title can be interpreted lots of ways.  First, frogs are amphibians.  They can live in both water and land, and they are adaptable, like humans.  They are fun to watch, and create laughs.  Since this is a satirical play about the many flaws and foibles of humankind, it seems to fit.

Another interpretation could be that this is a Biblical allusion.  The play was written in a time of chaos and crisis, and in Egypt during the time of the Pharoah Rames II, frogs were unleashed on the city in order to convince him to allow the Hebrew slaves to leave.  As another time of crisis, this could also be applied.

The title could also be symbolic of Aeschylus ridicule of Euripedes' plays--which Aeschyles says have caused a moral and social decline.  How better to illustrate this decline than to call all the people who watch and follow these plays "frogs"?