Last month, Siliconera ran an article with some talk from Capcom's Seth Killian, admitting that some people internal to Capcom of Japan didn't view Mega Man as being especially popular in the U.S. Since then, a thread at Capcom Unity was opened on the matter and both Christian Svensson and Seth Killian have elaborated further. Svensson states:
Relative to Japan, North American sales of MM titles is historically lower (though MM9 and MM10 were significantly higher in NA than Japan). Another complicating factor is the fact that the demographic for MM there is much younger than it is here. My suspicion is that it's due to the fact that for years, MM in Japan has been promoted in the weekly Kuro comics magazine, which is younger skewing and the Battle Network/Starforce series was also aimed as a much younger demo than Classic, X or Zero franchises, so we tend to get some mismatches across the brand.
We know it's a popular brand in NA, but perspective is all relative.
While Killian adds:
It was my quote so I should try and provide a little background...
As Sven mentioned, MM sales in Japan are typically higher, so there's that.
Beyond sales however, it's largely a cultural impression issue. I think most CJ staff view MM as a fairly "Japanese" character (as opposed to, say, Frank West). I think Americans often assume that because something is popular in America, everyone else probably likes it too. In Japan, it's usually the opposite--the more "Japanese" something might be, the less they would expect it to be popular elsewhere.
It's less to do with any reality and more to do with "this is a very Japanese thing, therefore foreigners won't be as interested in it." That's true for some Japanese things (*cough* natto *cough*), but not true for other things, like the Blue Bomber. It's easy for everyone to love a cute, tenacious, and pure-hearted hero with such fun enemies. Although just to make it even more complicated, MM is much less popular in Europe than he is in N. America. Of course Europe is just wrong about this :p, but different strokes for different folks...
What to make of all this? To one end, I can see Mega Man titles with more RPG influence being more popular in Japan overall. But I also believe that the core theme and idea to Mega Man is something that transcends a particular culture. A loveable robot hero is something that everyone can get behind without much trouble. There is likely a lot more working underneath the issue than mere nationality which creates the perception. But absolutely, I've no doubt Mega Man is still very popular in the U.S. Besides, we can take it upon ourselves to show how popular we think he is!