Not counting intentionally funny or humerous songs (such as you would typically hear on Dr. Demento) my favorite “one-liners” or single lines in songs that otherwise are not really funny are:
From “Has Anybody Seen My Gal?”
That always cracks me up. YMMV.
From “The Shoop Shoop Song” or “It’s In His Kiss” or whatever the real title is:
If you wanna know if he loves you so, it’s in his kiss (that’s where it is).
Probably not what you had in mind, but my demented mind finds these buried treasures of comedy endlessly amusing.
One last one; not really a funny line in and of itself, but it’s such a non-sequiter that it also cracks me up every time:
From “Happy Together”
After the song is mostly done, they repeat “Happy Together” and “So happy Together” several times in the chorus, and then add in
Well my dog died just yesterday and left me all alone,
the finance company dropped by today and repossessed my home.
But that’s just a drop in the bucket compared to losing you,
and I’m down to seeds and stems again too.
well willard’s knocked out on a bottle of heat,
drivin’ dangerous curves across the dirty sheets,
he said man you ought to see her,
when her parents are gone,
man you ought to hear her when the siren’s on.
you got to tell me brave captain,
why are the wicked so strong,
how do the angels get to sleep,
when the devil leaves the porchlight on.