Believe it or not there is a blog devoted entirely to this unusual subject. The comments accompanying the images may be on the juvenile side but the paintings are real.
More than a decade ago, my sister and I suffered from paroxysms of hysterical laughter in front of a Biblical painting containing an incongruously mature looking Baby Jesus in Mary's lap. The infant reminded us of an adult of our acquaintance. Our faces red and tears running down our cheeks by the efforts to suppress the hilarity we attracted the attention of a museum guard nearby who looked askance at our unseemly behavior. We evaded eviction by quickly moving on to another gallery.
There may be a cultural / religious explanation for why indeed some of the babies in Madonna paintings were so homely. I do not know and neither does the blogger as is clear from the following comment on page 3.
chicagonorth asked: it's inaccurate to call this "ugly Renaissance babies," because most of the images are from Byzantine/Medieval/Proto-Renaissance periods...
This is very true! Though, in our defense, a blog called “Ugly Byzantine/Medieval/Proto-Renaissance Babies” doesn’t really have the same ring to it.
Regardless of the specifics, I think we can all agree that ugly babies are both timeless and hilarious.
Perhaps someone with knowledge of this artistic phenomenon will leave an explanation in the comments section. (link via Anna Levine)