Marlow is suggesting that people who pretend to be honest are often the least honest.
The play is largely about deceit and sleight of hand. Tony tricked Hastings and Marlow into going to the wrong house. Marlow is shy around ladies, but has fun flirting with barmaids.
But I don't know how, a single glance from a pair of fine eyes has totally overset my resolution. An impudent fellow may counterfeit modesty; but I'll be hanged if a modest man can ever counterfeit impudence. (Act II)
Kate thinks that Marlow is reserved, when actually he is only reserved around her. When he does not think he is the presence of the upper class he is quite boisterous. It is also Tony’s fault, because Tony tricked Marlow. Marlow does love Kate though, in the end, and they marry.
The irony is that while Marlow is actually a more honest man than Tony, Kate is not as honest as she seems. Kate and Marlow are made for each other, but they have to play parts in order to see that. It is not until they got to know each other's fake roles that they could like each other.