I doubt PB is simons fiancée betty.
Pebbles might have a personality similar to betty, and smart like her as well so it might be one of the reasons ice king is drawn to her. Simon in the special called betty a princess, so it explains why ice king is so hell bent to marrying a princess, and a smart one like betty.
There is one more princess that could be betty, it’s Doctor Princess(or Dr.Princess) http://adventuretime.wikia.com/wiki/Dr._Princess
who looks ALOT more like betty than PB. Even says princess is just a name that stuck, so she might of just gotten the name after a 1000 years of being alive?
THIS SHOW KILLS MY HEART.