In short... yes, but that follows the general trend of reddit. This is the readership map /u/brigantus put together for our 200k census (we had a screw up, as people will recall, in the last census, resulting in a lot of people from countries on the top of the dropdown list), but as you can see from a search for general reddit democraphics, a strong preponderance of Americans is common for the site, generally. It isn't even close too. To quote from there:

3. 56% Of Reddit’s Visitors Are From The United States

Reddit is most popular in the U.S., with American users far outnumbering those from any other country. Second to the U.S., the United Kingdom accounts for 7.3% of all Reddit visitors and Canada accounts for 6.2%.

Now, that said, while the site is predominantly American, does your second point necessarily follow? Certainly, it impacts the volume of submissions, as you can see in this chart, but does it impact the attention? I don't believe that is the case. I'm currently working on the "2017 in Review" stuff, which will be posted soon, but one thing I've looked at is correlations, and there isn't a noticeable correlation between post time and score (=CORREL - 0.004538385) or post time and comments (=CORREL - -0.006016135). As the chart shows, there is a decline in volume, which might give the impression, but those threads themselves don't seem to be underperforming based simply on their time posted (although I'd note the time that they are getting their comments isn't included in my data, so it might not be happening until the US wakes up).

I imagine that's also because Reddit skews to the English language, so English speakers will be disoroportionate. Naturally, the numerically superior Americans dominate. I wonder how popular questions would compare if responders were all German, for example.