Do mirrorless cameras have lenses with automatic diaphragms like DSLRs or are you able to focus and view through the working aperture because of their electronic viewfinders?
It varies. Even when using the same camera, the lense can affect this feature, so if it's important to you, you're going to need to do your research. On my camera for example, three of my lenses can do DOF preview, but one of them can't for some unknown reason. This is not a standard feature and may be available or unavailable depending on the camera body, lense, and other settings.