Should an exterior surface-mounted outlet box on wood lap siding be flashed?
I don't think using a flashing will hurt anything; but I also don't think it is necessary if you properly install a weatherproof box.
You should consider potential cut/scrape hazard scenario as people (e.g.you) will be plugging things in and out, placing body parts near sharp sheet metal flashing. Also, make sure it will not interfere with the weatherproof box cover doors/flaps and/or any in-use covers you might use.
Also, I think that technically you need to feed the wire into the back of the box through a short conduit with a bushing (bushing to protect wire from damage where it enters the conduit), because of the wood siding...