Mozart described the importance of good operatic texts in a letter regarding The Abduction from the Seraglio (Die Entführung aus dem Serail) in a letter to his father in 1781:

…the Poesie is totally in tune with the character if this stupid, coarse, and malicious Osmin [the servant character] – and I am well aware that the kind of verse used here is not the best – but it agrees completely with the musical ideas that had been wandering around my head, even before I had seen the text, so I couldn’t help liking it; – and I’m willing to bet that when it is performed, nothing will be found inadequate. As far as the Poesie of the original play is concerned, I cannot say anything against it either. Belmont’s aria, “Oh, how anxious,” etc., could not have been written any better for the music. – Konstanze’s aria is not bad either, especially the first part, except for the bui and “sorrow now rests in my breast,” – for you cannot say sorrow “rests.” – I don’t know, but it seems to me – that in an opera the Poesie must always be the obedient daughter of the Music. – Why are Italian comic operas so popular everywhere? – in spite of their wretched texts! … Because the music reigns supreme – which makes one forget everything else. – and an opera that is well designed must, therefore, please all the more; where words are written expressly for the Music and not merely to suit some miserable rhyme here and there; god knows, they contribute nothing, no matter what they offer, to the success of a theatrical performance, but they can certainly do a lot of harm; I’m talking abut creating words – or entire strophes – which ruin the composer’s entire concept. – Verses are probably the most indispensable element for music – but rhymes – created solely for the sake of rhyming – are the most detrimental. – those gentlemen who work in such a pedantic fashion will always go under, together with their music. –

It is so much better if a good composer, who understands something about the stage and can make a suggestion here and there, is able to team up with an intelligent Poet and create a true Phoenix. – In such a case one need not worry about the applause even of the ignorant! – The Poets seem to me almost like trumpeters with their professional tricks! – if we composers always just follow our rules, which were quite useful in years past when one didn’t know any better, we would come up with a kind of music that is just as useless as their librettos.

– W. A. Mozart, in a letter to his father, October 13 1781.

Cited in: Spaethling, Robert (2000) Mozart’s Letters; Mozart’s Life. London: Faber and Faber, p.288-289.