To put you at ease, I also agree that in the vast majority of cases, an interview (even a mediocre one) will have no effect.
My experience (as a 25+ year interviewer) is this: for most kids, it won’t make ANY difference. The applicants are in three groups: a) shoo-ins (recruited athletes, other “gotta haves” that, barring a felony or something, will receive an offer) – this is only a very small group (2-3%), b) the vast number who don’t get much traction past the first read. I can imagine 85%. Then there’s c) that remaining 10% or so which they need to whittle down by 3/4 — the kids in the grey area. For them, an very good interview MIGHT make a difference.
For group a) and b) who represent the vast bulk of applicants – there is no effect whatsoever.
That being said, I know of students in the “c” category for whom the alumna’s report did tip the scale in the applicant’s favor in the committee’s eyes.
I veer from @yikesyikesyikes completely cynical view of things (at least from my first hand knowledge with my school). It may hold true for some but not mine.