The phrase "mostly concerned about" is correct and usable in written English.
It can be used when expressing a primary focus or worry regarding a particular issue or topic.
Example: "The committee is mostly concerned about the environmental impact of the new project."
Alternatives: "primarily worried about" or "mainly focused on".
Primary concerns varied by region: African students were mostly concerned about financial aid, Middle Eastern students about campus safety and Asian students about career prospects.
I am mostly concerned about usage and security.
These middlemen are mostly concerned about price and quality, not the welfare of foreign workers.
I am mostly concerned about comfort and safety; my husband with the logistics of the trip, and what sights to see.
In the article, he explains, "I am mostly concerned about second- and third-tier universities and colleges, and community colleges, many of which play critical roles for education and economic development in their local regions and communities".
"The issue he was mostly concerned about was that he really did not want to attract controversy from the violent elements of various animal rights groups.
These errors mostly concerned peaks for which shape was atypical.
Moms Demand Action, a network of mostly women concerned about America's lax gun laws in the wake of tragedies such as the Newtown elementary school disaster, have been waging a successful campaign to persuade big chains to prohibit the carrying of rifles into their stores.
However, he remains concerned about the 600 mostly elderly residents who have been reluctant to leave their homes in about five or six high-rise apartment buildings in Atlantic City, our colleague Jennifer Preston reports.
If you answered mostly D, then yes, you've probably got something to be very concerned about.
They were all really concerned, mostly about his behavior, so they wanted help with... containing him somehow.
