How would the poem we wearing the mask be defined of Paul Laurence Dunbar had written and more informal language or without as much structure?

This is a great question. The key argument to be made here is probably that the structure of the poem itself can be equated with the "mask" which the speaker says we wear at all times. A formal structure like the one Dunbar chooses here makes use of traditionally accepted elements, such as a regular rhyme scheme and the use of archaic language, like "nay" and "O." This immediately casts the poem into a form we recognize, because we have seen many poems structured and pitched in this way before. As such, the structure not only lends cohesion to the poem, it also confines it.

How Dunbar's poem might have looked without the "mask" of such a formal poetic structure and use of language, we cannot say. However, it is very probable that the way Dunbar writes here is not the way he would have expressed his message in spoken language. As such, he is not taking off the mask in protesting about it, but rather using the "mask" of traditional poetic form to indicate how confined we are by that mask.