emojis are actually built into base lemmy for the past year or two (contributed by a dev affiliated with hexbear, but still), but they don't federate seamlessly and most instances don't use them much, or at all, so app support also seems to be lacking last I saw.

If you're using lemmy.ml on the web not thru an app, you can still try out the emoji picker/autocomplete, it just only has standard unicode emojis. Try typing :a into the comment box or clicking on the little smiley face in the bar above the box!