You'd have to ask the originator of the expression for their intended meaning. To me, it sounds like a general purpose form of abuse.
However, if we were going to be deconstructive about it one could start by asking what we know about wombles. What are they known for?
Basically, they wander unseen across Wimbledon common collecting rubbish (the everyday things that people leave behind.... or something like that). So generally, some kind of secretive public area activity.
Then consider our notions of 'cock'. Clearly it could relate to fowl or poultry, but equally to the male sexual organ (also used for urinating). Add those senses to our knowledge of wombles.... and the idea that is most alive to us, the sense we most clearly have is that of something akin to a.... chicken herder. QED.
A cockwomble is a chicken herder.