In the barest sense of the word, "empire" implies that one country uses force against another. No one is forced to adopt US culture. People do it voluntarily, or not at all. We might be a "cultural empire" if we purposefully emigrated mass numbers of people into a region in order to populate the area with Americans and thereby make the area American. We don't do this, that I know of.

No one is forced to buy goods from the US. If people buy them, it's because our goods are something they want. Trade is a mutual agreement to exchange goods. The US is under no obligation to trade with anyone, and no one is under any obligation to trade with the US. If our trade policies influence other nations, it's because they allow it to happen, or because they're not as good as us at producing something. This doesn't equate to force.