Is Mr. Gilmer's cross examination of Tom Robinson skilful?

No it isn't skilful. Mr. Gilmer uses derogatory language like "boy" to Tom Robinson. He uses a very sarcastic tone to demean Tom Robinson than actually question him,

"Mr. Gilmer smiled grimly at the jury. 'You're a mighty good fellow, it seems- did all this for not one penny?'

'Yes suh. I felt right sorry for her, she seemed to try more'n the rest of 'em-'

'You felt sorry for her, you felt sorry for her?' Mr. Gilmer seemed ready to rise to the ceiling."

Mr. Gilmer's cross-examination was not clever or technical but it might have been effective appealing to the bigotry of the jury.