It's easily breakable after baked? It's not a very thin layer . . .
In the case of polymer clay, "thin" is a relative term unfortunately!
Re the strength/brittleness of baked clay, thin can actually be anything that's not a rounded ball or oval! (those are inherently strong shapes no matter what they're made from). Even 3/4" of thickness would be weak if it were large enough vs. its thickness
And the 3 Sculpeys (S, SS, SIII) are just more brittle after baking than other brands of polymer clay which is something that has to be considered for certain types of projects (...they also have a "harder" surface, which is nice, but must give up strength and flexibility for that characteristic).
The biggest factor then is the stress
a clay item will receive after baking. If it simply sits on a wall or a desk or something like that and is never stressed with use or wearing, it will be fine (similar to greenware). But if it's used, or pressed on, etc, or if the material it may be covering or attached to is flexed or swells even a small amount, then the thinner the Sculpey is, the more likely it is to snap off or chip.
So covering an Altoid tin with Sculpey (even 1/4" or 1/2" thick) will make that covering much more susceptible to breakage if stressed than covering it with another brand would have been. Stronger clays, if properly baked, etc., won't break at all without tremendous stress, and they'd bend or bounce before breaking (those would be clays like Premo, FimoClassic, Kato, Cernit, and probably FimoSoft).
(btw, "thinness" also applies to any projecting
areas which are thin at any point... so for example, if you dropped this tin on a hard surface the stars which project out though just a a little, could chip on their corners in particular.)