They don't check when you get off the bus, just when you get on. So if you packed everything as checked luggage, and only had park necessities in your carry on, you could simply get off the first stop your DME bus gets to and get on an Epcot bus there. All your checked luggage would still be delivered to your hotel, and you just keep your carry on with you in the park. (IE backback) That could save you 20-30 minutes depending on what bus stop your resort ends up being on the route. Assuming your flight arrives on time, you are quick to deplane, and don't make any stops in the airport I think you might make it. But if your arrival is delayed, or you they just dispatched a bus to your resort, you might miss. I'd guess its usually 30 mins from gate to DME, another 15 mins for bus, 45 mins to WDW. Then another 15 mins to wait for Epcot bus, and 20 mins transport, you would make your 12:15. But shifting any of those times, you would miss. If in doubt, either drop the money on an Uber/Taxi or reschedule your reservation.
Trying to bring luggage through security into the park (where the lockers are) would be a pain (if they even let you do it), and I wouldn't recommend it.