At the beginning of the movie, Saleri's caretakers attempt to coax him out of his room with some kind of frosted cinnamon bun or croissant. Granted he definitely seems to be very wealthy, but was something like that readily available? Did they have icing?
I guess I've always imagined elaborate desserts in that time to be something you wouldn't just have laying around like a box of Pop Tarts today.
The movie "Amadus" is fiction, but some aspects of it's depiction of daily life in the 1780s are reasonably authentic (for Hollywood, anyway).
A frosted paistry like that seen in the movie would have been an occasional luxury, rather than an everyday treat like Pop-Tarts or Hostess are today. For one thing, sugar was much more expensive, relatively speaking. That said, Saleri's household wasn't exactly poor, and by the late 18th century various forms of raw sugar and molasses could be purchaes from dry-goods merchants in most towns of any size.
They certainly would have been able to make a variety of icings and glazes -- all you need is sugar and fat (butter, lard, oil, etc). In a city with the size and wealth, there might very well have been a specialty pastry baker at which you could order such a delicacy.
So basically, sugared pastry wasn't "readily avilaible" the way is it is today, but if you had the kind of wealth Saleri is shown with (big house, silver candlesticks, servants, etc), then a sugared cake would have been an attainable luxury, but not an everyday snack. That's why the servants are presenting it as something special -- not only was the sugar a bit pricy, in the days before electric mixers and pre-heating ovens, making proper pastry dough was very time-consuming and required skills that not every cook had.