The eclectic clientele and the zany decor have kept UpStairs on the Square a go-to institution for 10 years, and for 20 years before that around the corner in Harvard University’s Hasty Pudding Club. Today, surprisingly, many customers are sealing business deals and, not surprisingly, proposing engagements, conducting weddings, and celebrating graduations and bar mitzvahs. The whimsy is a stark contrast from the slick design sensibility that defines many restaurants, and the formality of classic wood-paneled steakhouses. But for longtime patrons, that’s part of the draw.