Thanks for bringing this up! Tags on mobile are only available when creating posts (either a reply or posting a question). On the desktop version of the Community, you can add tags to posts through the page layout without editing.
I do not have any information on if this will be available in the future, but I can look into it with our vendor to see if this is something that has been suggested yet to them. If it hasn't, I will make sure to put in the suggestion to them.
Please feel free to let me know if there are any other features you would like to see on mobile or in the Community overall.