30 years ago I worked as a reptile keeper for a medium-sized zoo. We occasionally got calls from people with snakes in their yards, wanting to know if they were dangerous. One such call concerned two large snakes a man had spotted while looking over his fence into a neighbor's garden. I asked him to describe them to me, and when he did, I figured he was a prank caller, because no snake in that region matched the descriptions. Then his out-of-breath wife returned from having gone next door to warn the neighbors, and informed her husband that they were just inflatable snakes set out to scare the birds away. We all had a good laugh over that one.
As far as being a way to protect figs, in my experience many birds will actually approach snakes (as well as owls and hawks) quite closely, to harass them and try to drive them away (a behavior known as "mobbing"). My guess would be if the plastic critters were realistic enough to fool the birds, you might end up with even more feathered friends in the yard (or nosy neighbors peering over your fence)!
The only ways I've found to reliably keeps birds from eating my figs are bird netting "tents," covering individual branch ends with fabric sleeves, or putting little newspaper covers over individual figs and clipping them in place with clothespins.