In real life, in his last (surviving) letter, Mozart wrote:
… at 6 o’clock I fetched Salieri and Madame [Catarina] Cavalieri with a carriage and took them to my box [in the theatre where The Magic Flute was being performed] … Salieri listened and watched with great attention, and from the overture all the way through to the final chorus there was not a single number that did not elicit from him a “bravo” or “bello”. He and Cavalieri went on and on thanking me for doing them such a great favour. [Robert Spaethling (trans and ed), Mozart’s Letters, Mozart’s Life, (2000)]
Of course, he just might have been faking it!
Wikipedia calls the play “a highly fictionalised account” and says that Peter Shaffer ”used artistic licence in his portrayals”. I haven’t found anything from Shaffer himself on the topic. (See: https://en.wikipedia.org/wiki/Amadeus#Historical_accuracy.)