It's not just a Jim Carrey movie. Before the days of simply wearing them for Halloween, masks were often used as a symbol for deception, hypocrisy, and lies. Dunbar's poem is no different. The speaker refers to them, directly and indirectly, as the reason why black Americans and people in general are unable to speak honestly about their suffering. He's not talking about a real mask of course, but rather it's a symbolic one that represents the things people say and do that aren't honest. But Dunbar also reminds us that masks are sometimes a crucial part of self-preservation, bearing in mind the dangers that black Americans often faced if they chose the more honest route.
Title: We know that mask is at the heart of the poem and the problems the speaker addresses. Perhaps we can go even deeper to say that the entire poem, therefore, wears a "mask" since a title defines a poem.
Line 1: The opening words are not only the poem's refrain, but also get right to the point about what Dunbar's poem is about. Those masks may "grin," but they also "lie." So don't be fooled by that smile.
Line 2: The mask hides everything, including "cheeks" and "eyes" which symbolize the truer essence of humanity since we often express ourselves best in those places.
Lines 9, 15: The refrain reminds us just how pervasive that mask is. It's not enough to just mention it in the title. The speaker strategically reminds us throughout the rondeau that this is no small occurrence, and therefore needs to be emphasized.