Rubba-dub-dub doggerel

Hi,

"If you are hoping for an Arts Council grant for your rubba-dub-dub doggerel, you'd better keep the day job."

It's from the reader's response in The Independent to the comment of another reader about the the 'poem' put in the readers' comments:

"Old Git Tom
Was a fine old man
Taunts dem coppers
As far as he can

Old Git Tom
Never did run
Faced dem coppers
And had some fun

Old Git Tom
Earned a bruise or two
Fought dem coppers
While the crowd grew

Old Git Tom
Saw deir gloom
Watch dem coppers
Run as fast as a koon

Old Git Tom
Heard everyone cheer
No more coppers
No more fear

Old Git Tom
Fell face down in de mud
One of dem coppers
Left him ridd’n with blood."

My question is: is the noun "rubba-dub-dub doggerel" a neologism meaning 'nonsense' or is it a well-established word in the English vocabulary meaning something else?

Thank you.