Are Minecraft statistics and achievement stored somewhere on Mojang servers, or are they stored only locally on my computer? For example - if I logged into Minecraft on my friends computer, will statistics and achievements be "synced" and visible there? If not, does someone know if it's somewhere on devs to-do list?
They are stored locally, as explained on the Minecraft Wiki:
You can reset all of your Achievements by deleting the "stats" folder in the .minecraft app data folder. (WARNING: Also resets Statistics)