Ok. Since the other two users that answered are basically telling you that it's actually allright for Germans to say "heil _____", let me give you the other perspective.
It is definitely not allright. Now, if you say it as a joke, you will probably not go to prison for it. Though if you are unlucky you might get a fine. What will definitely happen however is that (at least reasonable) people will think you are a moron.
The reasoning behind that is not as suggested in the other answers. It is neither about shame nor about forcing or encouraging people to have a bad conscience for something they haven't done. Not primarily anyway.
Since the end of the Nazi Regime the word "Heil" was no longer used as a greeting (in most regions of Germany/Austria) except if you wanted to express your affiliation/support for Nazi ideology. People with Nazi ideology will still use "Heil" as a greeting from time to time (when they want to make a statement about their ideology and affiliation. Not in public of course, since most of them are afraid of being generally known as a Nazi and the possible social and judical repercussions that come along with that.) So when you use the expression "Heil" you are using the same expression Neo-Nazis (modern Nazis) will use. I'd advice against affiliation with Neo-Nazis, unless ofc you feel that way (in which case you are a c***).
That is part of the reason why people will think you are a moron. Now obviously it was a joke, so why all the fuss? Well, the other part of the reason is quite simply that jokes affiliating yourself with Nazi ideology and jokes where you only mimic Nazis (without actually ridiculing them) are simply bad taste. The Nazis did horrible things, murdered millions of people. You don't joke about that stuff. Let's say something horrible had happened to the mother of a good friend of yours, let's say she was murdered by gangsters or maybe raped. Would you then go around and make lots of jokes about people getting murdered/raped? Would you tell your friend jokes about murderers and rapists? Would you go to him make some fake gang symbols and yell "Hey, look! I am a gangster. Hahaha.. Oh come on, it was only a joke! Relax duuude!" Well maybe you would. But I wouldn't.
So when making jokes regarding Nazis there is a fine line seperating "well that's still ok to say" and "what a dumb s*** would ever say something like that". Different people draw the line differently. My opinion is: Making fun of Nazis is ok. These aholes don't deserve to be protected from ridicule. But if your making "fun of them" solely consists of dressing up as a Nazi, making the Nazi salute or saying Nazi slogans, I honestly don't get how that is riduculing them or how that is funny in any way. It is about the same level of humour as dressing up as a rapist for a Halloween party and going around yelling whatever you think rapists would yell. Who would do that shit? Well, only morons.
Well that text got a bit long, sorry. I hope it helps you to understand better.