Hello, everyone.
I believe that you are referring to the following feature which is available on the desktop & web app:
On the mobile app, you do not see the smiley icon to choose the emojis. Can I please confirm if I get that right?
If yes, the emoji picker is only available when you type in the :. This may be a good feature request to add next to the camera icon (used to snap photo) in the mobile app. You can raise this up in the Mattermost Uservoice channel for public visibility.