I travel to Utsunomiya maybe twice a week for work and I see many foreigners (mostly from Asian countries).
According to the city's home page, about 30% of all the foreigners in Utsunomiya is Chinese. Followed by people from Brazil, Phillipines, Korea and Thailand. Less than 3% are from USA. As of 2006, out of 509,509 residents, 8,039 were foreigners.
Is it "cosmopolitan"? I don't know. It IS Tochigi after all.... so it's no Tokyo or Osaka.
I suggest you take a day trip out here and see it yourself. By Shinkansen, it takes less than an hour from Tokyo.