Has Dan Stevens moved from “Downton Abbey” too soon? On the same day the show’s recent season was anointed the highest-rated PBS series ever, Stevens — who won’t be back next year as Matthew Crawley — hit a Hugo Boss bash Tuesday at the Guggenheim for artist Danh Vo, with guests Jeff Koons and Marian Goodman. “No one really recognized him,” said a spy of Stevens, who’s been sporting short dark locks and facial hair. “And you’d think the Guggenheim crowd would all be big ‘Downton’ fans.”