From the Mangadex description:
https://mangadex.org/genre/12
"A harem includes three or more characters who potentially show romantic interest in a male protagonist. The sex, gender, or orientation of the harem members is irrelevant as long as they exclusively, or at least primarily, are vying for the affections of the same individual - who may or may not reciprocate towards one, several, or none of these romantic rivals. (Source: Anime-Planet)"
I don't see how Riko is primarily vying for him and again if the logic is just a few characters blushing around the MC makes a harem then the tag should be applied to nearly every shounen series on this site.
The MC of Sousei no Onmyouji/Twin Star Exorcists has at least 4 characters with crushes on him and it isn't tagged as harem and that's a major mainstream series, same with Bleach.
Just doesn't feel like the tag is being used in a consistent way.