The picture you linked shows a strong rangefinder spot, but it does have indeed weak contrast. Which likely means that the main viewfinder-prism is fine, but it's hard to make any judgement beyond that.
Shine a light into the viewfinder system, from the back of the eyepiece. Look at the RF window as it "lights up". Do you see any cloudiness or dirt? In that case it likely can be cleaned. If it already looks clean, the RF prism could have desilvered or deteriorated. In which case a CLA without a replacement part will not do much.
In a nutshell it's possible that a CLA will improve things, but it's hard to say without more info. We could give you more information if you told us about the type/make of your camera.