Was it more common to use "im" back then, as opposed to today where we would rather use "am". Or was that just him being extra spicy?
Can't speak to historic usage, but today "am" (an dem) means "on" or "near", and "im" (in dem) means "in".So the literal translation would be "lick me in the arse".
It's true that Mozart disliked her. His own letters confirm that. It's unproven if he wrote the soprano voice that way to make her bob her head.https://www.snopes.com/fact-check/mozart-head-bobbing-soprano/
Mozart invents headbanging.