Whatever value this product has for marketing is outweighed by it's potential for abuse—stalking and harassment.
When an Instagram user wants updates about when his ex posts now, for instance, he has to be following her. So she knows he can watch her posts and has the power to block him.
InstaMon completely circumvents what little control people have in these situations, and is only contributing to a world in which one bad actor can force a person (usually from a marginalized group) out of the public online square. ("Because if you don't want to be stalked, you can always lock your account.")
It's clear from his comments the creator doesn't see the issue and doesn't have a plan for dealing with abuse. But when you're creating someone that automates existing behaviors on a platform and one of those behaviors is abuse, it's your responsibility to have a plan for addressing it.