If a person always speaks ill of his mother-in-law in front of others, what will others think? Shouldn't you say mother-in-law in front of outsiders?

Of course, this kind of thing is best said less.

Because the mother-in-law is your elder after all, and can't always do something wrong. If you say that your mother-in-law is not good in front of others every day, others are likely to think that you are not easy to serve, but your impression will get worse.

The most important thing is that what you say will reach your mother-in-law's ears, and when she comes to quarrel with you, your relationship will get worse and worse.