Lufthansa business class ID90
 

Is it acceptable to wear jeans?

Personally, I would say no, out of respect. When on id90 I always wore decent trousers with jacket and tie, admittedly in the 80's and 90's. Decent dress will mostly get you on a flight, if possible and also maybe in First. But then times have changed.

If it's long haul and you're well dressed you will stick out like a sore thumb ;) As Zed3 mentions, it was once the case that there was a fairly strict dress code for ID pax, especially in Business or First. It was easy to pick out who were the ID pax and who were the fare paying pax! I know that in latter years things were relaxed up to a point and I believe jeans are accepted as long as they are not scruffy or ripped etc. I think that jacket and tie is no longer a requirement but t-shirts are not accepted. I'm no longer acquainted with LH dress code. Why not contact their staff travel office and ask them.

Smart casual will work very well. Ties are almost unheard of these days. I still would prefer chinos over jeans.

There is a staff travel office to contact?
Obviously, jacket, dress shirt, cool shoes, and stylish, neat jeans are on the agenda lol....

Ah, I see you're in the USA. Then contact LH at your intended airport of departure and ask them (to be sure).