After a jewish couple gets married and they go to the yichud room, is the choson allowed to kiss the kallah?
Nit'e Gavriel, Nisuin 1, chapter 37, is about the Yichud room and he mentions no prohibition on kissing that I see.
The whole point of the yichud room is to provide seclusion of sufficient duration so that the marriage could technically be consummated. Traditionally we don't do so, but you certainly could. And kissing is certainly not a problem.
Interesting side note - some traditions invert the order of the yichud room so that instead of taking place after the chuppah, it takes place after the meal (when everyone has pretty much left). The reasoning is that once there is yichud and we consider them fully married, the wife must cover her hair. By shifting it to the end of the wedding feast it allows her to remain with her hair uncovered during the dancing and the meal.