I hate that. Why do people lock their Miis?
I think some people just forget. I have a total of 38 or 40 Miis I meticulously worked on to fill my apartments with, and I can say from experience that after the 8th or 9th Mii I started to forget to click on the "Other Settings" tab when filling out name information for that Mii and thus leaving certain Miis locked. I would say out of all of the ones I made I have roughly 30 of them unlocked.