I agree that any time of the year is a good time to visit Japan. I do agree that Hokkaido in winter may have fewer outdoor activities, and snow may make travel within Hokkaido during winter to be more time consuming. But I do not think winter Hokkaido is only for skiers. There are winter delicacies available at Hokkaido.

シャコ (shako): mantis shrimp (Jan. - March)
八角 (hakkaku): poacher fish (Nov. - March)
ニシン (nishinn): herring (Jan. - March)
たらこ (tarako): cod roe (Nov. - March)
毛ガニ (kegani): hairy crab (Dec. - Feb.)

Drift ice on northeast coast of Hokkaido is winter event. Icebreaker ship take tourists off the coast for the tour of the ocean covered with drift ice. JR Hokkaido has a tourist train Ryuhyo Monogatari during winter season to see drift ice from the land. JR Hokkaido also runs a tourist train SL Fuyu-no-Shitsugen for the tour of winter wetland near Kushiro. Then there is famous winter snow festival at Sapporo. I do not think you should leave out winter Hokkaido if you are not a skier.