Mixing valve for bath
Is that normal? Yes.
Common? Yes. You'll find plumbing in a similar state in probably hundreds of thousands of homes in the united states.
It is a problem waiting to happen? Yes.
Is it dirty? No.
That bad boy right there is good and corroded. A good candidate for proactive replacement. In the future, whether you wait for a leak (which could be a blow out or could be a pinhead) or you go ahead with replacing now, find a valve with solid brass connections to play nice with the copper. whenever you go to replace, give that adjacent copper a good look over to see if any of it looks like you need to cut it back and replace with new.