What is the ruling on lying to one's mother about permissible matters, then gradually confessing, in order to avoid her anger, and is the one who does so considered a liar?

It is recommended to be kind and good to one's mother, for pleasing her is from pleasing Allah. If informing her of something would upset her, then one can use ma'areed (euphemisms/double entendres) to avoid lying. These are truthful statements from which the listener understands something other than what is intended, such as saying "A Muslim is the brother of a Muslim" in a specific situation. The Prophet, peace and blessings be upon him, said: "Indeed, in ma'areed there is room to avoid lying." If telling the truth leads to a predominant harm, then there is no sin in lying, although using ma'areed is preferable.