Is it permissible to supplicate by saying: "O Allah, convey my greetings to my mother"?

Greeting the deceased is a Sunnah, and it has been established that the Prophet, peace and blessings be upon him, used to greet them and teach his companions to do so, as mentioned in the Hadith of Aisha, may Allah be pleased with her. The deceased receive the greetings of the living and return them, so there is no harm in supplicating that Allah convey your greetings to your mother. It is best to adhere to what has been established in the Sunnah.