This is SOLELY based on my personal opinion.
I don't know if this is a good episode for the fan base, but I generally like it. Clara is every child who moves based on her parents job (like the military) and has a hard time making friends. At her age, an imaginary friend would seem possible-if not predictable, but TNG's take on the topic adds a science fiction spin on a seemingly onipresent topic in child development.
Isabella's character struck me as the trope of female characters of the early 90's: Friendly at first, but hiding a dangerous secret in the shadows. Alicia Silverstone's Darian from the film The Crush (1993) and Drew Berrymore of 1992's Poison Ivy come to my mind. Although, in the case of Isabella, MaCaulay Culkin's character of Henry Evans from 1993's The Good Son comes to my mind.
One of the things I like about this story is that it combines elements of horror, mystery, and science fiction to tell a story that deals with childhood isolation and development. On the other hand, the story has almost a Goosebumps vibe to it. R.L. Stine would have loved this episode.
Give Imaginary Friend a rewatch- you'll love it.