Ignoring a problem never solves it. You need to place yourself in your MIL's shoes and try to figure out the reason why she may be "bullying" you. If she bothers you (i.e. taunts etc.) then you need to talk to her and find out what's bothering her so you can work on that.
I dont think there is any reason for why MILS do this :D I believe its an issue of how they think DILs can never be good enough for their son. If its not one thing, its the other.