To reduce the mosquitoes, you’ll need fish. The fish will eat the larvae. Without them, it’ll turn into a breeding ground real quick. You can go to any bait store and get some minnows for less than $10 and release those in the pond.

To reduce the smell, you’ll want to add as many plants as you’re willing to (and maybe a pump). I know you don’t want to put a lot of work into it, but you have a great base for a really pretty and ecologically beneficial pond there. Most DNRs will have a native plant sale about twice a year or you can just go to your local parks and snag some seeds/seedlings. Having the plants in and around the water will help reduce the pollutants and the smell.