Yeah, it’s a bit strange.
One of the strangest reasons I’ve heard about why Starlight and Trixie are there is that “the Mane 6 have too many episodes, you can’t do anything special with them anymore”, which I disagree. I think they should receive very interesting and creative episodes. Besides, many of the filler episodes where the Mane 6 are the main characters (especially in season 6) only perpetuates the problem and it reinforces that defensive attitude from the people who like or love Starlight and Trixie.
The only episode where Starlight was the main character and I kinda cared was A Hearth’s Warming Tail, from season 6. When I finished the episode I thought “maybe Starlight wouldn’t be a strange addition to the main cast”. She would get interesting episodes where she would learn from her mistakes.
Then I watched Every little thing she does and I changed my mind.
But nowadays I don’t mind that too much. I admit I get a bit intense with two episodes: That one and the season 6 finale. But other than that, not too much. I tend to say that the inconsistency of Friendship is Magic is more than enough reason to not take it seriously.
Besides, if the show was actually consistent, Starlight would have been an interesting character instead of one of the most toxic characters I’ve seen in a modern TV show in many many years.