Can you bird mouth cut rafter to the inside of wallplate

Probably, but will you lose any overhang for a soffit?

A birds mouth is designed to hold wall at a specific location.

If u are cutting rafter without an overhang, u have to make a seat cut to sit on top of wall.

A birds mouth that is cut from inside of wall will only weaken the rafter.

A birds mouth that starts at inside of wall and is cut toward the ridge board serves no perpose.

Chuck