The idiom, plonk (something/someone) down means
- to slap something down; to plop something down
- to sit or lie down on something in a careless or noisy way
- to leave someone somewhere to do this; Dave plonked the kids in front of the TV and disappeared upstairs.
- to put something down heavily and without taking care: Just plonk the shopping (down) on the table, and come and have a cup of tea. Come in and plonk yourselves (down) (= sit down) anywhere you like.
From these various definitions I can surmise why cheap wine is often called plonk, it's the sound of the bottle slapping down heavily on the table.
But how did we get from that to "a plonker" which basically means a silly or stupid person.
"Why did you do that, you plonker?"
Nicholas Lyndhurst; Only Fools and Horses