Aunt Polly tells Tom that he can do a good job when he’s of a mind to. What was she talking about?

Aunt Polly was referring to the newly whitewashed fence..... although she didn't know that Tom hadn't been the one to paint it.

Aunt Polly placed small trust in such evidence. She went out to see for herself; and she would have been content to find twenty per cent. of Tom’s statement true. When she found the entire fence white-washed, and not only whitewashed but elaborately coated and recoated, and even a streak added to the ground, her astonishment was almost unspeakable. She said:

“Well, I never! There’s no getting round it, you can work when you’re a mind to, Tom.”