The spurious ‘of’ in phrases of the form ‘it's not even that funny of a joke’

This seems to me to perhaps be a pattern of speech from certain parts of the country, and nothing at all to do with grammar. Wot w(h)ere I come from, there are all manner of bizarre constructions, none of which behove to natural rules. Or sense. (My pet hate is the 'would of'/'would have' confusion, but with the accent, you can understand it.)

What you are really saying is that you don't like country folk... ;)

If I heard that construction from a "country folk" type person in amongst a lot of other country dialect, I probably wouldn't think twice about it. However, when I see it written by someone who I know speaks standard English, I don't think that argument holds up.