I see it as part of Willie's contradictory nature. He often says two opposite things at the same time. Remember in Act 1, he says that Biff is a lazy bum and then says: one thing about Biff, he's not lazy.
This is beyond confusion; he is constantly conflicted in the things he believes. He is the ultimate salesman who believes his own sales pitch, yet knows it's a sales pitch.
Part of him respects people who work with their hands. He is proud of all the work he has done on his house... the stoop he has made and all the repairs he has done. He says he know every nail. Indeed, he has poignant memories of his father who produced and sold his own hand-made flutes... thus the sound of the flute as the play begins.
Willy is torn and misled by the modern world in which he lives... a world that at once reveres the honest work of one's hands yet pays far better the one who wheels and deals.
Seems like a silly thing to be proud of, doesn't it? But there is logic behind why Willy is so proud of the ceiling he has installed. You have to remember that Willy is not happy with the idea of being a carpenter, even though he would be good at it. He instead chose to spend his life struggling as a businessman under the misguided notion that a lousy businessman was more important than a great handyman.
Charley, the neighbor, is good at being a businessman and is more successful at the things Willy wants to be successful at. The one thing that he is not good at is carpentry. So when Willy does a good job installing the ceiling he is very proud of himself because he knows that it was something Charley could never do.