So, the moral is, if you:
(a) are extremely sensitive and fussy about your own physical comfort, and
(b) complain to your host about small imperfections, instead of being tactful and polite
then this means that you are special and deserve to marry a handsome prince and live happily ever after.
Somehow I cannot imagine the princess from this story living "happily ever after". If she's that sensitive to a pea under 20 mattresses, then how will she react when the honeymoon is over and the prince starts hogging the duvet or leaving the toilet seat up?