I want to build a little pond for the frog in my backyard, but I don't want to spend to much time and effort maintaining it and I dont want to run a pump or filter in it. So if I put some little fish in there will they eat the mossies and pollution? How can I keep it clean?

It will be a pond that will naturally dry up sometimes and wont always be full of water.
Sorry, I mean. if it can be a pond that will dry up sometimes, will that help to keep pollution and mossies out?

Fish will cause additional pollution, not clean it up. If you aren't willing to put the time and effort into maintaining it, forget it.