Why Tirpitz was most useful in the Norwegian fjords

Would Tirpitz have been more useful if Germany had not kept her in the Norwegian fjords?

No; the fjords maximized her value as a protected fleet-in-being threatening the Arctic convoy route.

The fjords placed Tirpitz close enough to menace Allied convoy planning while making her difficult to attack. Until Britain could employ long-range aircraft and carriers confidently in the area, the anchorage offered unusual protection for a German capital ship. This is classic fleet-in-being logic: the possibility of a sortie forces an opponent to retain covering forces even when the battleship stays in port. The panel argues that moving her elsewhere would sacrifice much of that leverage. A surviving Bismarck alongside Tirpitz would have made the northern threat still harder to contain.