Flowers have sprouted

The sparrows sing cheerfully

Cool mornings warm nights

There once was some meat on my plate

I thought the steak and ribs were just great

But I got indigestion

It was ugly no question

It wasn’t the best meal for a date

I think the format of limericks, AABBA, does influence them being comical.  I find that the first two lines are longer in length and lead into the story of the poem.  The third and fourth lines are short and have the comical aspect to the story.  Wrapping up the story with a longer line like the first two lines and ending with the same sound as those lines also adds to the comical nature of the poem.  Limericks seem witty to me because you have to be able to tell a story in five lines where three lines end in one sound, and two lines end in another.

There is just something about the format;

Longer line A

Longer line A

Shorter line B

Shorter line B

Longer line A

that makes these poems seem comical.