It's hated because it's a 90 minute advertisement targeted at little kids that shows how out of touch Sony execs are with pretty much every demographic imaginable.
Sure, we get films with advertising all the time, but rarely so blatant is it that a film is made to advertise than the Emoji Movie. That it's targeted at young children means that it's targeted at the one demographic that's unlikely to be able to discern advertisement from film, and that's pretty fucking irresponsible. On top of that, you've got insincere lip-service paid to any number of social movements in an attempt to make the movie far more than it is. Finally, just on an artistic level, there's no shortage of both visual and written plagiarism.
The Emoji Movie is honestly pretty reprehensible on most levels you can look at it on. Bob's confusing contrarianism with being a voice of reason. Yes, people shouldn't pay attention to the film, but to suggest it is only hated because 'EW KIDS STUFF' is to ignore the fact that Disney movies ostensibly aimed at kids still make gangbusters in the same periphery young adult demographics that hate the Emoji Movie so hard.