Based on the positions of the Know-Nothing Party, which was characterized by its strong anti-immigrant sentiment and nativist policies, the conclusion that can be drawn from Millard Fillmore receiving 21 percent of the popular vote in the 1856 election is:
A significant number of voters were hostile toward immigrants.
The Know-Nothing Party primarily focused on restricting immigration and promoting the interests of native-born Americans, which aligns with the observed support for Fillmore's candidacy.