The phrase "mostly dirty" is grammatically correct and can be used in written English.
It is typically used to describe something that is primarily or mostly unclean or covered in dirt. It can also be used figuratively to describe a situation or scenario that is unpleasant or morally questionable. Example: "The abandoned house was mostly dirty, with dust-covered floors and grimy windows, but there was one room that seemed oddly well-maintained."
"My jokes are mostly dirty," he had said in the green room.
Mostly dirty south, in other words: hip-hop that isn't encumbered by crippling reverence towards its old-skool past.
From a middling distance, in contrast to the noble-looking mountain, it is mostly dirty, dun-gray scree.
Further, a type of 'quantificational variability effect' may be observed when a quantificational adverb (mostly) is combined with a minimum standard adjective in sentences like "The dishes are mostly dirty", which may receive either a degree interpretation (e.g., 80% dirty) or a quantity interpretation (e.g., 80% of the dishes are dirty).
Most space dust comprises rock (rich in silicon and iron) and ice (mostly dirty water), but a good deal of it also includes carbon, especially a class of organic compounds known by the tongue-twisting name of polycyclic aromatic hydrocarbons (or PAHs, for short), similar to the large benzene-ringed molecules found in cigarette smoke and automobile exhaust.
By this he means mainly short stories, but there are also essays, reminiscences, four pages of "party jokes," 21 mostly dirty cartoons and a long interview in which Saul Bellow is asked what he thought about the O. J. trial ("in California the whole justice system is in deep trouble") and whether he has known any actors intimately (only Marilyn Monroe, and not in that sense).
The jobs that automation has replaced have mostly been dirty, dangerous, or disagreeable.
And the scholars who know the most about language have mostly shunned dirty words as a subject.
The custom-made brick facade, once the color of burning coal, is mostly a dirty, weathered red.
Instead, he found that there were very few beggars about - mostly very dirty little boys - and there were people who went out of their way to be kind and helpful.
A few loads from helicopters, part of a last-ditch haul to cover the snowboarding and freestyle-skiing sites at Cypress, were mostly too dirty to use for the pipe, Petrie said.
