I suspect Lerner had a brief attack of the Yip Harburgs when he tried on this bit of "peculiarish" not so "grandish" whimsy!
Funny you should say that, because my first instinct when reading your comments on "aren'" was to think of the horn/your'n rhyme in "The Great Come And Get It Day."
Normally, rhymes like that would be fine, IMO, in the spirit of having fun with the language. Ira Gershwin did a lot of that too. But for Higgins to be doing that doesn't make a lot of sense at all. (If the guy actually had a sense of humor about language it might be different...but he clearly doesn'.) :-)