In the original Spanish, the aphorism is "No ser malila", "not to be malila". For some reason the 17th century Italian translator of the aphorism decided that the corresponding expression in Italian was "not to be the nine of tarocchi". This engenders two puzzles. One is, what was this Spanish game and/or card "malilla"? And what is this game in which the 'nine of tarocchi" is equivalent? And is it an Italian game similar to Malilla, or simply the Spanish game in translation?
"Malilla" means "bad one" in Spanish, so "bad card". But bad for whom? Online I find various interpretations. The most comprehensive is probably https://es.thefreedictionary.com/malilla, citing the Grand Diccionario de la langue Espanola
It is meanings 3 and 4 in the first dictionary, and the first meaning in the second dictionary, that are relevant. What fits the explanation in Vitali's source is meaning 4, because of the verb "servir". But the nine is only mentioned in meaning 3 of the first dictionary and the second meaning of the second dictionary.malilla
1. s. f. JUEGOS Carta de la baraja que, en algunos juegos, es la segunda en valor. mala
2. JUEGOS El dos de cada palo, en el juego del rentoy.
3. JUEGOS Juego de cartas en que la carta segunda en valor es el nueve de cada palo.
4. Lo que sirve para todo, como un comodín.
Gran Diccionario de la Lengua Española © 2016 Larousse Editorial, S.L.
f. Carta que es la segunda en valor en ciertos juegos de naipes.
Juego de naipes en que la carta superior es el nueve.
Diccionario Enciclopédico Vox 1. © 2009 Larousse Editorial.
1. s. f. GAMES Card of the deck that, in some games, is the second in value. bad
2. GAMES The two of each suit, in the game of rentoy.
3. GAMES Card game in which the second card in value is nine of each suit.
4. What works for everything, like a wild card.
Great Dictionary of the Spanish Language © 2016 Larousse Editorial, S.L.
F. Card that is the second in value in certain card games.
Card game in which the top card is nine.
Encyclopedic Dictionary Vox 1. © 2009 Larousse Editorial,)
But what does any of this have to do with the game of tarocchi in Italian? Could the "malilla" perhaps be the nine of a trump suit?
Another source, https://www.pagat.com/manille/malilla.html, has the malilla as "the seven of a non-led suit", which cannot be played in a trick, and so is a bad card in the sense of being unlawful. Perhaps originally it was played with a 48 card deck instead of a 40 card deck as currently.; then it would be the nine. There is a trump suit in that game, that of whatever the last card dealt is. But what such a card, a nine or seven of trumps, would have to do with the aphorism is totally unclear. But there is a trump suit in that game. So perhaps it is a variant of that game to which the aphorism refers.
The Malilla then ought to be the highest card in the trump suit, which in Italian might have been called "tarrochi", perhaps an old word for "briscola", meaning "trumps". Such a card would "serve in every situation", as the book explains of the malilla,by winning, and be "bad" for the players that don't have it. Alternatively, the nine could be a wild card, which, like the Fool, could be played in any trick and not win any, bad in that sense. But I find no verification of either.
To complicate matters, one of Vitali's sources says it is the nine of coins! Could there have been a game in which Coins was the permanent trump suit, or perhaps the highest or lowest suit, in a game in which the suits are ranked?