Are all mother-in-laws bad and annoying? If so, why?

No, not all mother - in - laws are bad and annoying, there are many mother - in - laws that are nice to their child’s spouse.

Don’t let the popular notion that mother - in - laws are ferocious and annoying scare you. Many of them are actually of great help to their child’s spouse. They are there to put you through and let you know many things you didn’t know about your spouse. They offer great advice that can help you grow your relationship with your partner.
Do not run from her, embrace her, show are love and receive all the love and help she’s willing to offer to you.

No, mothers in law are not always bad and annoying. Otherwise, that would be like saying the majority of women who have married children are bad and annoying. What you are experiencing is no doubt a conflict of power.

Who has the most influence over the loved one? Who does s/he listen to the most, and take their viewpoint? It is very hard when you marry and assume that your loved one will have no ears for anyone's opinions and wishes than yours. Thee are othe people in your partner's universe, as there are in yours.

The mother has been of vital importance during childhood and she will naturally feel the loss when her adult child now turns to someone else. Just treat you mother in law with respect and try to discuss differences of opinion wih her if they affect your partner.