The phrase "are mostly full" is correct and usable in written English.
It can be used to describe a situation where something, such as a container or space, is primarily occupied or filled to a significant extent.
Example: "The parking lot is quite busy today; the spaces are mostly full by noon."
Alternatives: "are largely occupied" or "are primarily filled".
Except for morning and evening rushes, when trains are mostly full, runs are typically half occupied or less.
Some of the new highways may be empty, but Kroeber, the editor of the China Economic Quarterly, said the high-speed trains are mostly full.
The company are mostly full of attack, lines snapping into place with verve, and they always look like they're having a lot of fun.
Yet the shelves are mostly full, thanks to single rows of the same few products – Royal brand paper towels, Zulia beer, Lina sardines, Heinz spaghetti sauce – which are repeated in aisle after aisle.
The areas north of Fordham Road, and to the west toward the University Heights neighborhood along the Harlem River, are mostly full of five- and six-story brick apartment buildings, many with Dominican and Puerto Rican flags hanging from their fire escapes.
Although the popular image may be that of a Caribbean Casablanca harboring bon vivants with a penchant for fast cars and faster cash, the streets of downtown George Town, on Grand Cayman, are mostly full of gawking tourists from the cruise ships anchored nearby.
"The profile in our country is still that hockey players are mostly full-time workers.
Open the unit, slide two panels into place, and you end up with a keyboard whose letter and number keys are mostly full-size.
Because our cDNA resources are mostly full-length cDNAs including the TSS and the polyA site, complete sequencing of these cDNA clones will add to our understanding of the mRNA diversity.
