first of all, "傻瓜" is a noun, which means you have to say "somebody is a 傻瓜"; "笨" is an adjective, which means you have to say "somebody is 笨"。However, they are of similar meanings: to say that somebody is stupid or silly. I guess the only difference is that sometimes we use 傻瓜 to close friends/your boyfriend or girlfriend/ you children when they make some obvious mistakes, but we mean no harm; however, if you use "笨", it sounds a bit offensive.