In Twelfth Night what makes Sir Andrew and Malvolio so easy to trick?
Malvolio's arrogance is what makes him so easy to trick. He believes himself so superior to everyone else that he genuinely thinks that Olivia is in love with him, despite her higher social status. Malvolio is so lacking in self-awareness—another characteristic that makes him so easy to trick—that he thinks he's entitled to better things. This makes him easy prey to Maria's crafty plot to humiliate him; she's going to teach him a lesson as punishment for his constant nagging, criticism, and condescension.
As for Sir Andrew Aguecheek, well, he's just a fool. Not a fool like Feste, a court jester kind of fool, but a fool as in a complete idiot. Like the hapless Malvolio, he thinks he has a chance of marrying Olivia. And like Malvolio, he's sorely deluded. But so obsessed does he become with securing Olivia's hand in marriage that he's the perfect patsy for the cruel prank that Sir Toby Belch plays on him.
Sir Toby has convinced Sir Andrew that the best way to Olivia's heart is to act the part of a swaggering blade and pick a fight with "Cesario." But Sir Andrew, being a physical coward as well as a complete imbecile, takes fright when "Cesario" (really Viola in disguise) looks ready to run him through in a duel. So it's not just congenital idiocy that makes Sir Andrew so easy to trick, but also his seemingly limitless capacity for macho posturing.