Marshmallow creme can be really sticky when you're using it in a recipe. But, in this case, you're not actually touching it (or, at least, you shouldn't be). You're just putting the marshmallow creme on a knife, then using that knife to spread it over the pretzel. Since the pretzel is warm, it may try to slide a little if you put on too much at once. Just grab it with the knife and move it back to the top of the pretzel. When you do that, you may notice the marshmallow creme isn't smooth anymore, but just give it a couple seconds and it will smooth itself out.