It would be great to have an emoji picker, or at least have it auto complete when tying “:”. Feature idea is here http://mattermost.uservoice.com/forums/306457-general/suggestions/10335798-emoji-picker. Please vote it up.
Though this thread is old, I wanted to make sure that the information is shared with everyone. As of now, I can confirm that the Mattermost version 5.4.0 includes the emoji picker after typing a colon and at least two characters as addressed in the MM-4992: Open emoji autocomplete after typing two characters. For example:
Hope that helps.