I think that word niche, in this case, is being used as a crutch for a game that doesn't need one. It's a fail safe word. Like, you can't blame the wii fanbase if it fails, it's a niche game. The same thing was done for zack and wiki (though a bit more appropriately in that case, being that it is point and click). Rez is a niche game, as is katamari damacy. New (unorthodox) approaches to new (or less common) ideas. NMH is a new (unorthodox) approach to a very common idea (linear action/adventure).

However, with that said, it is a good game and I'm very happy it's selling.

The Wii is a monster both hardware and software wise. I think this game WILL be a millon seller. 

Well, the first hurdle for this game is the 500k mark. Once it reaches that level, it can be considered a "convincing success". If it does reach 1 million, then we can say this game is no longer niche and that GTA has a competitor.

100k for this game in less than a month is good.

100,000 * $50 = $5,000,000

There was little marketing so figure the publishers and developers get a good portion of that maybe $30-35. $3,000,000 probably recovers most of the cost to the dev and the pub.

While 100k isn't great even for a niche title, the sales pattern of No More Heroes is pretty interesting. It's sold virtually the same amount for the three weeks it's been out.

Looks to me like retailers weren't expecting it to sell at all, and are now trying hard to get more stock. Hard of course being relative to how little they cared about it in the first place.

great i hope it continues to sell very well

 NMH is NOT a GTA clone

I would assume that by now world wide, the game has sold at least 180 - 200K to retailers (130 K to consumer), meaning you could almost double what you calculated. Please keep in mind the actual money publisher and developer make is based on games shipped to retail. 

it is not out in Europe, but we don't hear/read anything about it. except when you are really looking for it yourself.
I always find it very strange that there is no marketing at all for some titles.
Or even worse that you can't find the games in the shop.
I from holland and I had to import viewtiful joe from the UK, the same with Ico (although the re-release i saw on the shelves). And NMH will probably be the same.

 Not only is it a good game it is an AMAZING game. It's graphics aren't going ot wow you but I genuinely found myself wanting to uncover the next part of the story or see the next weapon. It is a truely satisfying game that deserves to succeed if for no other reason than the ending that hints at a part 2!