I think that, as far as when emotes can be considered harassment, a lack of forethought has placed a number of emotes into the game which hardly have any other purpose than to be offensive. I'm certain that the design team thought that they had simply made cute or funny choices in what to include, or at the very least that they had simply picked out selections which matched the current theme of the Prize Map but unfortunately it doesn't seem that any more thought or consideration was put into them beyond that so now the "poop" emote is the standard insult to be hurled or paired with other expressions to form insults or derogatory statements.
On a broader sense though, if we set aside emotes which automatically have negative associations, anything at all can be considered harassment if you have the right mindset, including the most innocent of expressions, so it can be really subjective. For example, if someone runs a vanquish streak on a player, like 0-9, and emotes the happy face every time somebody out there is going to feel targeted and label that as some sort of sarcastic harassment.
Having said that though I do think that there are emotes which were added which were phenomenally poor decisions on the part of the design team considering that they were being introduced into a competitive game environment, which is remarkably different from using emotes on your phone.
