Yes, the UI is dynamic. You'd need Super Admin rights to even attempt a change which is only @Bozza you'd need to not break the dynamism or mobile look and feel and have somewhere to change and deploy the code, however simple.
Might be a five minute job, might be a PITA. I'm not the man who can...
Hmm. Desktop / laptop shows date thread was started on. By default mobile shows time last updated.
I honestly don't know if it will be "very quick to change", especially if the mobile responsive design is Xenforo's out of the box UI. I will have a poke around and after that you're in the hands...