What insight does Gratiano offer about how a person’s appearance can be misleading?

It's act 1, scene 1 of The Merchant of Venice, and Gratiano is doing his level best to cheer up his despondent pal, Antonio. He cheerfully offers to play the fool to help Antonio take his mind off his mounting business troubles. Antonio has told Gratiano that he feels like he's playing a part on the human stage, the part of a sad man.
In response, Gratiano tells his friend that appearances can be deceptive. Although Antonio may look sad, that's not the man Gratiano knows and loves. Gratiano says he knows lots of men who go around with stern expressions on their faces, and people think them incredibly wise. But that's only because they never open their mouths. If they did, says Gratiano, then they'd probably make complete fools of themselves and everyone would be able to see what they're really like:

There are a sort of men whose visages Do cream and mantle like a standing pond, And do a willful stillness entertain With purpose to be dressed in an opinion Of wisdom, gravity, profound conceit, As who should say, “I am Sir Oracle, And when I ope my lips, let no dog bark!” O my Antonio, I do know of these That therefore only are reputed wise For saying nothing, when I am very sure If they should speak, would almost damn those ears Which, hearing them, would call their brothers fools.