This question is already discussed here, where emojis are seen as noise and several issues of displaying emojis on different devices are seen as a drawback.
How should we handle emojis and are they a reason for editing a post?
Yes they are allowed, if a post can be improved by editing, it's allowed and even welcome. Of course, what is an improvement can be subjective (spelling errors are objectively bad IMO). At this moment, I'm not aware of any post from which I would remove the emojis. Let's reconsider when emoji-use becomes rampant.