A backslapper is someone who's overly excited about congratulating you. Sometimes a backslapper's enthusiasm seems insincere or exaggerated.
If you're a backslapper, you go out of your way to offer hearty compliments or effusive good wishes to other people, especially if you want to impress them. Some people criticize Hollywood for being a culture of backslappers who endlessly compliment each other. The word comes from the custom of literally patting or slapping someone's back as a way of congratulating or encouraging them.