Mars has 1/8th the mass of the Earth. The mass of all the main belt asteroids adds up to a fraction of the mass of the Earth’s Moon, which is itself 1/100th the mass of the Earth. IOW, this terraforming idea won’t work.
Martian habitats for humans would/will consist of orbital stations and closed, self-sufficient buildings on the surface.
Pity about the asteroids, but we could still drop a couple comets on Mars to add water and gases to thicken up the atmosphere.