chrism: The problem with your viewfinder is most likely a tarnished mirror. You can get a new one here:
http://www.uscamera.com/yashicatlr1005.htm
It's an easy swap- remove the hood (4 screws), push down on the little tab of metal in the middle of the upper edge, and push the old mirror out. While you are in there, clean the back of the lens since it is probably coated with evaporated grease from the focus bearings. The new mirror (front-surface so do not touch. I wear gloves and handle only by edges) can be slid into place. Since it is located by cast parts in the body, and both new and old mirror are front-surfaced, there is no need to adjust focus.
Assuming that your hood has the Fresnel screen, this is two pieces. Glass on top (what you see when focusing) and plastic on the bottom, inside. If you are up for disassembly, press the clips down and slide sideways.... this may make sense when you are in there. Use dish washing soap on the Fresnel, and on the ground glass to avoid removing guidelines.. Note that the screen is not symmetrical! Both lines and the center clear spot of the Fresnel are off-center. Make notes/marks. Be sure everything is dry before reassembly.
If the screen is still disappointing, Rick Oleson will sell you a new one:
http://rick_oleson.tripod.com/index-175.html
You will see on his site that he says that his screens and the stock Yashica-Mat will not show much difference. My experience says to start with a new mirror, clean the back of the viewing lens, a cleaning of the ground glass if you are up for it, and see what happens.
Anyhow, about a year ago I bought a Yashica-D to try medium format again. I hadn't shot film for more than 20 years when I started again in the spring of 2009. I used to have a Rolleicord, and the Yashica seemed a nice return. I lucked into a late model D with the Yashinon lenses and the 2.8 viewing lens. Since then, I have bought 5 more Yashicas of various models (all with Yashinons). Right now I have a D and aYashica-Mat with Lumaxar lenses. I am in the middle of getting another Lumaxar ready to sell. I tend to use my Minolta Autocords theses days, but every time I pull out one of the Yashicas I am impressed.
They are great cameras, hitting a sweet spot between price and quality. Probably the best way to see what medium format is really about. And given their price, you can do things like this without too big a grimace- a Hasselblad NC-2 prism finder scabbed onto a Yashica-Mat (go to Previous for another view)-
http://www.flickr.com/photos/180672...4647805714/in/set-72157624555271098/lightbox/