"mostly part of" is not a grammatically correct phrase in written English.
A correct way to phrase it could be "mostly a part of." It can be used in a sentence to describe something that is mostly included in or associated with something else. For example: "Being outgoing is mostly a part of my personality, but I can be shy in certain situations."
There were other composers' string quartets before Haydn's great ones, but they were, mostly, part of a tradition of household music, and played for private enjoyment.
The beaches of Portugal's Alentejo region are mostly part of the south-west Alentejo and Vicentina Coast natural park, making this one of Europe's best-preserved coastlines.
There are some crescents with the crosses and Stars of David in the upper reaches, but they seem mostly part of a celestial decorative scheme.
She supports the building of a museum in Berlin to commemorate the expulsion of millions of Germans from her country's eastern territories (now mostly part of Poland) after the war.
The community is between a stretch of highway and the Rio Grande and is bordered by mountains and desert that's mostly part of Ted Turner's 358,643-acre Armendaris Ranch.
Re "The Dark Side of Liberation Artsts pages, May 21), about a book by Mary Louise Roberts asserting that sexual assaults by American soldiers were common at the end of World War II: The millions of soldiers who served in World War II were mostly part of a civilian Army — young men fresh out of high school, in college or newly married, dedicated to doing their part in ending Hitler's rampage.
Mountains are mostly parts of the upthrust rims of ancient impact basins.
"As for the plane debris, it was mostly parts of the plane's upholstery made of plastic.
Her face was mostly bare — part of the defiance of the broadcast, and the risk.
It is just a mostly inescapable part of getting an interview with an artist or celebrity".
These are common nowadays on SUVs and luxury cars, mostly as part of expensive option packages.
