The last time I saw her mother she was still a beautiful woman.
However before posting here I looked up her bio. During the time she was married to John Phillips she says she (they all) used mind altering drugs. She had extramarital affairs, and helped to create a bad environment for the kids that were exposed to the lifestyle. I don't know how many kids there were in the group, but in the bio Michelle says the Mamas and Papas lived a communal lifestyle.
The children would have seen the adults doing drugs, and had access to them. They didn't have any responsible adults in their lives. Kids learn by example.