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

Gratiano tells Antonio that men who look stoic and wise, avoid talking because they would show themselves to be fools if they said anything,

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.