What you're doing is watching nature run its natural course. I think you were wise to do nothing.
The baby will probably move to a different location in the shrubs, be very still and wait out the night. Then in the morning Mama will protect it again as it strengthens its wings. In a day or two.....off to wild blue.
However, if this particular bird doesn't survive, then it will have one or two brothers or sisters that will.
It's not unusual for a pair of mockingbirds to have two, sometimes three broods in a season.
What's that phrase in Disney's "Song of the South"? Twitterpainting!!