'Pear-shaped' is an RAF euphemism for 'tits-up', which is itself a euphemism for 'dead'.
The Oxford dictionary definition cites an RAF source in 1983. I was an RAF pilot in 1983. The reason no-one will confirm the meaning officially is that it's rude, hence the rubbish about aerobatics. Loops that go wrong are egg-shaped, and have always been referred to as such.
The only shape that has no fruit connected to it is the hourglass shape.
Body parts like breasts can be compared to shapes too:
Melons (full and round), pear shaped (half full, most people have this), pointy (where the nipple sticks out), hangtits (hanging banana's), or flats (no tits)
Also may relate to a popular French Revolution caricature of "citizen King" Louis-Philippe, drawn by Honore Daumier and Charles Philipon in 1832 as a jowly pear. The French term "poire," or pear, was slang for "fool" or "idiot" - and soon thereafter, all caricatures of the King were banned, pear-shaped or not.