Translation:The mirror is in a corner of the room.
Not at all. For German "a" and "one" are the same. The ending of "ein" is just about the case and gender of the ensuing noun. Use these two words how you would normally. If you are not a native English speaker, as I am, the difference between "one" and "a" might be difficult to grasp. I think both should work here. If not report it. Or wait for English speakers to correct me.
Native English speaker here. Exactly how is it "einer Ecke"? The mirror can only be in one corner, it can't be in "all" corners. I can imagine the sentence to read "Der Spiegel ist in einem der Ecke des Zimmers". Or, can you combine "in einem der Ecke" to read "in einer Ecke"? Very confusing to be honest.