I think I actually know this one!
I've heard progressive writers talking about 'The Piper', named after the main character of Orange is the New Black: a main character who is white, middle class and most importantly, boring. (in their opinion) That is, safe and anodyne, not a character who will turn off potential viewers but ultimately a bait-and-switch character that the show will devote increasingly little time to in order to spend time on more 'interesting' people. Usually poc, queer, disabled etc, though I guess really the only prerequisite is that they're perceived as lacking mainstream appeal and thus need to be snuck in under the radar.
I don't think Finn started this way, but after Ward left the show I think the writers very quickly lumped him into this category.