Yes, I think "ain't" is correct English (much more so than "less items"). It's not formal, but then neither is "fucking isn't". It's also localised in usage, but then so is saying "treacle" to mean "syrup" (or vice versa if you prefer). All are genuine English, as commonly spoken and understood by English people, and consistently described in dictionaries.

Disclaimer: grew up in Essex. Have observed teachers trying to inform pupils that not just their word usage, but their accent is "incorrect".

Further disclaimer: I do it myself, and hence can't really criticise it in others, but I think that often when people say "X is incorrect English", what they really mean is "I wish X were incorrect English". I prefer to think in terms of "better" and "worse" use of language where possible: English does not have an authoritative definition, so where there is genuine disagreement the most one can say is that one side of the debate communicates more effectively than the other.

Next: double negatives for fun, but not no profit.