It doesn’t meet your standard for a funny punchline. It is clearly a punchline though. It’s a weird mischaracterization to say that this comic is without a punchline.
That doesn’t ring true. It is clearly a punch line, easily identified by both of us. You just don’t like it. You find it weak and bland like you do most of her work.
It is really a punchline. Also it isn’t predicated on the reader’s reaction being transphobia. The reader could simply be aware that transphobia exists and is a common reaction to the familiar situation she depicted. You don’t need to self insert as literally one of the characters in the comics you read. She isn’t saying that you are transphobe or that she is incredible. She is making a comment about how parents and society’s reaction to a trope situation is changing.