The only thing that I would bring in regarding emoji use in display names is #a11y issues if there are a lot of them. I'd say recommended practice should be to have 1 and max. 2 emoji in the name, so #screenreaders don't have read a long shebang you sometimes see on 'funny' display names.
But perhaps someone better versed in #accessibility issues for the visually impaired can chime in on this #ActivityPub #FEP (and perhaps other FEP's too), if this is a real issue. A "fediverse accessibility best-practices" may exist as well (somewhere).