Sir John's fault is in being indiscriminately "nice"--setting no limits on his gregariousness. He provides a home for the Dashwood ladies at an affordable rent, he keeps them stocked with game & garden stuff when they are not at his own table; he would love to see them set up with good husbands. He takes in the Steeles; he wants to keep everyone happy. I think it's ok to allow him some entertainment at supposed romances since he believes that everyone is as open & honorable as he is so he can't imagine duplicity or unkindness. He truly thinks everything will work out for the best. If he is childish it is only in being innocent & free of guile. He may not be the most scintillating conversationalist but he is a good man. I would enjoy having him as a friend--Ms.Armstrong, not so much.
I think he lets Lady Middleton's criticisms go in one ear & out the other without offence or alteration.