The MEAN stack is not, itself, an architecture. It's just a handful of tools that work pretty well together for building web applications. "Web Application" probably comes closest, but there's a lot of detail you can go down to determine the actual architecture.
That said, you will find a number of application frameworks which use the MEAN stack (e.g. MEAN.io, MeanJS) which in turn implement a software architecture of their choice.
One could argue that it defaults to at least a 3 tiered architecture, because it implies that you have a DB (Mongo), an application server (Express & Node), and a client app (Angular). But that's pretty thin, frankly, because you can still arrange things very differently even if you assume those four basic building blocks.