What is the principle of decorum in neoclassical poetry?

Decorum is the idea of politeness and uprightness in life, and by extension, poetry and literature. It was extremely prevalent in now-classical poetry and in a myriad of works from the time periods of Baroque, Victorian, and other similar styles of literature. There was a heavy emphasis on decorum and propriety in literature of this period.
The main aspects of these styles of literature is civility and high-class society. Because of this, the characters are typically portrayed with a great deal of decorum or are contrasted against the concept of decorum to show that they are not refined. With poetry, this is equally true, as decorum is used to show beauty, structure, and design, and poetry was written with highly effusive language.

In neoclassical poetry, decorum is the style and mannerism that is thought of as vital to a good and complete work. To have good decorum is to be in touch with social norms and propriety. The neoclassical poets considered reason to be the fountainhead of all knowledge and learning, and wanted poetry to reflect their desire for intellect and human understanding. The object of neoclassical poetry was to strike at the heart of universal truths that resonated with mankind as a whole, as opposed to any esoteric emotions that might appeal to a few. To this end, there were unspoken but strict and well-understood rules as to how poetry should be presented, and any works that deviated from these rules were considered to be in very poor form or, in other words, lacking decorum.

In reference to poetry, decorum refers to the fitness of a style to a subject matter. This means, for instance, that characters should speak in ways that match their class upbringing or that the Church should not be spoken of in vulgar language. A work with decorum will also not make light of tragedies by treating them in comic ways and will have a consistent use of diction throughout.
Neoclassical poetry generally adheres to the rules of Classical poetry, which regarded decorum as essential. Neoclassical poetry seeks to communicate a message about life or morality, and because of this, adherence to ideas of decorum in Neoclassical poetry also serves to reinforce social expectations that the author might hope to emphasize or make a point about.

Decorum can be defined as behavior that is in keeping with good taste and propriety. It was a very important concept in neo-classical poetry. The general consensus in the 18th century was that poetry should be written according to certain rules and standards. As such, there was a prevailing distrust of anything unusual, particular, or in any way innovative. Poetry should speak to common human experience and not simply be the eccentric outpourings of a tortured individual. Emotions certainly had their place in poetry, but only those emotions that are widely shared, understood and recognized; and even then they should be expressed with appropriate decorum and restraint. Otherwise, poets were little better than religious cranks and fanatics who claimed to have a unique inner light, a divine inspiration that revealed to them the absolute truth.
For Pope, as for many in the 18th century, the proper study of mankind is man. And poetry should concern itself with uncovering universal truths that speak to man's reason. Inevitably, this meant that poetry needed to be written according to an objective standard, one that could be understood and appreciated by anyone exercising their reasoning faculties. Hence the need for commonly-accepted rules of taste and propriety against which poems, as with all works of art, should be judged.