Bottom-tier overall, yes they are. Worst? Nope, they are not. Situational. Like others said, maps with lots of islands and archipelagoes - is the thing. Lots of turnda, too (though it's truth normally no player would go much tundra willingly). However, on any map with indeed _large_ tundra areas - those norwegian troops shine quite much, with that extra movement and +25% combat power. The upgrade, i believe, should stick with unit's upgrades to stronger infantries, too - all the way to mechanized infantry, making 'em stronger and - in tundra and hills - 3 times faster than Xcom squads, without any need for Uranium, too (thinking GDRs, that is).
Like many other civs, certain circumstances are needed for Denmark to become strong choice. Some civs shine in deserts, others in forests, etc etc; Denmark happen to shine on a map much or most of which is tundra and lots of islands, with that. Extremely rare, usually, circumstances - given OP's settings for the game (usual settings, i mean). So yes, if said settings are to stay, then the best thing is to start over with some other civ, i guess...
But if one would make a map of, say, imaginable planet - always near-zero cold all-around, thus all tundra, with no big continents whatsoever, - that's when Denmark would be strong.