https://screenrant.com/young-sheldon-season-6-why-big-bang-theory-mary-so-bitter-different/

The events of Young Sheldon season 6 secretly explain why Mary Cooper was mostly bitter when she visited Pasadena on The Big Bang Theory.

Why is Mary bitter?

Her husband died. The husband has never been "retconned" on Young Sheldon because George Senior has always been shown to be a very nice and good husband, father and family man. He also isnt a drunk, he drinks beer like Dan Conner on Roseanne/The Conners. The whole story of "Young Sheldon" is incomplete and maybe in the end, it will be shown that George Senior was a lout who had secret affairs. The problem is for the current show is that "Young Sheldon" wants George Senior to be likable.

Just saying, but most truly evangelical women like to have sex with their husbands because God has made it legal for them to do so. To leave the parents and (the man) cleve to his wife and all of that. It is alikely the Worldly legalization of marijuana. People can smoke marijuana and not be ashamed. Religious women have permission to have sex and will have it whole heartedly.

It hasnt been explained but maybe when George Senior died, the finances of the family became a bad problem due to George Senior's folly. But this hasnt been shown and I am not a writer on the show.

Missy turns into a rebellious teenager later in lore from The Big Bang Theory. This was brought out when Sheldon and Leonard went to Texas and went to Georgie's tire store, and Georgie in a heart to heart with Sheldon stated how bad things gotten when George Senior died and Missy's behavior. Missy has been married more than once and has more than one child and is pregnant on Big Bang Theory with another.