Is working under a state farm agent a good stepping stone to becoming an agent?

Any insurance related job is a good stepping stone.

Keep in mind that State Farm agents are captive, and therefore you will only learn about the products that that company offers.

Independent agents have access to a variety of different insurance companies and products, both preferred standard and non standard markets as well as Surplus Lines and International markets.

Farmers Insurance agents have a preferred market, as well as some limited ability to broker business to the surplus lines market.

Whatever route you take, be sure to get your insurance license and negotiate a commission split with the agent in addition to your hourly wage.