I have a 10 foot Sears jon boat. it is not super stable. Will adding a casting deck in the front be a good idea?

No!   The boat is designed to have the people sitting down in the boat. If you add a raised casting deck you will raise the center of gravity and make it more unsafe or unstable. If you do anything, consider adding some of the nicer seats that have clamp on bases like these. This is a seat assembly sold at Cabelas, but Bass Pro and others have these as well. Some are in camo, some come in grey or green, but they will make sitting in the boat more comfortable than on a bench

 

Not at all. 

And you would have better luck putting flotation devices on a picnic table