America absolutely should be considered an imperialist power during these years. While the country did not compete in the Scramble for Africa, it was very involved in gaining influence around the globe during this time.
The US acted imperialistically by trying to gain trading and other concessions in various countries. It forced Japan to open to trade. It demanded that it get concessions in China like those enjoyed by European nations. It forced Cuba to give the US veto power of Cuban policies.
The US also acted imperialistically by actually taking over various places. The US took Hawaii and parts of Samoa. The US took Puerto Rico, Guam, and the Philippines.
By taking power over so many places (formally or informally) the US was acting as an imperial power.