Mirrorless cameras and Night photography

Let's talk about lenses first.

You can get cheap lenses in almost any mount, mirrorless or (D)SLR. You can get very expensive lenses for any mount, mirrorless or SLR. You can get a lot of lenses in between for either type of camera. Since most SLR mounts have been around decades and most mirrorless mounts have only been around less than a decade, there are far more used lenses for popular SLR mounts (Canon EF, Nikon F, Minolta/Sony A, Pentax K, etc.) in circulation than for the newer mirrorless mounts. But for buying new lenses you need to look at the particular lens you are interested in and compare similar lenses from one platform to another.

Now let's talk about cameras.

In very low light the actual view through the viewfinder of any type of camera - film, DSLR, or mirrorless - can be very different than the resulting photo. This is more a function of the length of exposure normally used in very low light than anything else. Your eye observes the scene in real time through a viewfinder. If the camera is collecting the light from a period of several seconds or several minutes, there's no way to see that through the viewfinder near instantly.

But when we talk about a digital camera's efficiency, we're not usually talking about how much the view in the viewfinder matches the actual image produced. We're talking about what percentage of the light that enters the camera is converted to energy and recorded as a photograph. In theory there's absolutely no difference in how efficient a camera with a mirror can be versus a camera without a mirror. One can put the same exact sensors in either type of camera, and some manufacturers from time to time have done just that. For example, the Canon EOS R is a mirrorless camera that uses the same sensor that is provided in the Canon EOS 5D Mark IV which is a DLSR. So again, you're right back to comparing two specific sensors from two specific cameras, whether both are mirrorless, both are DSLRs, or you're looking at one of each.

Depending upon exactly what kind of "night photography" you plan to do Sony sensors may be better or less well equipped to handle your specific use case. For astrophotography some Sony models have a reputation as "star eaters" because their aggressive noise reduction routines eliminate dim stars along with digital noise. On the other hand, those same models with aggressive noise reduction might be just the thing for night street photography.