Pretty is used to tone down a statement and is in wide use across the whole spectrum of english See ‘meaning & use’ for definitions, usage, and quotation evidence. It is common in informal speech and writing but is neither rare nor wrong in serious discourse.
We can use pretty as an adverb, before an adjective or another adverb, meaning ‘quite, but not extremely’ There are 20 meanings listed in oed's entry for the word pretty, four of which are labelled obsolete … that's a pretty hat you're wearing
She's got such a pretty daughter She looked pretty in a simple cotton dress. Pleasing or attractive to the eye, as by delicacy or gracefulness. See examples of pretty used in a sentence.
When particularly stressed, the adverb pretty serves almost to diminish the adjective or adverb that it modifies, by emphasizing that there are greater levels of intensity. In a delicate or graceful way Pleasing or charming but not grand or overwhelming A pretty little cabin in the woods.
