I can tell you the fandom agreed with you! A lot of people just disliked how bland Iris was compared to the other characters and then there's the very significant objection to patient/therapist. It's to a degree that when you see requests for POI in exchanges they will often list John/Iris as something they DO NOT WANT!

... I have my own theory about why she's there but I'm not going to spring it on you before you've finished watching the show.