Mr. Boffin kind of like Winnie the Pooh: he's bumbling and naïve, but also loyal and kind. The whole reason Old Man Harmon leaves his money to Mr. Boffin (and Mrs. Boffin) is because Boffin was always his most loyal and trusted servant. When we first meet Boffin in this book, we find him trustworthy because, quite frankly, the guy doesn't seem smart enough to deceive anybody. He's also pretty baldly honest—he's even willing to admit that his wife is smarter and more stylish than himself, saying,
These arrangements is made by mutual consent between Mrs. Boffin and me. Mrs. Boffin, as I've mentioned, is a highflyer at Fashion; at present I'm not. (2.1.125)
On top of his naivety, Mr. Boffin is incredibly impressed by anyone with a solid education. He's bowled over by Mr. Rokesmith's first demonstration on how to write a proper letter, as the narrator notes,
Mr. Boffin was no less delighted; indeed, in his own bosom, he regarded both the composition itself and the device that had given birth to it, as a very remarkable monument of human ingenuity. (5.15.30)
But as the book unfolds, we realize that Mr. Boffin's kindness and lack of suspicion also make him an easy target for cheats and grifters like Silas Wegg.
Being rich seems to take its toll on Mr. Boffin as the plot of Our Mutual Friend unfolds. Over time, the man becomes suspicious of nearly everyone around him, especially Mr. Rokesmith. As Bella Wilfer says at one point,
Before my eyes he grows suspicious, capricious, hard, tyrannical, unjust. (11.4.125)
But guess what? This is all an act. Mr. Boffin is pretending to be mean because he's trying to help John Harmon find out whether Bella could ever marry Harmon for his love alone. We find out that Mr. Boffin was his old nice self all along when Mrs. Boffin says,
[If] you could have seen him of a night, at that time of it! The way he'd sit and chuckle over himself! (19.13.41)
Well, it looks like Mr. Boffin is as kind as he always was. By the end of the novel we feel bad about believing he was corrupt. But we were wrong about one thing: Boffin is clever enough to be deceptive.