"Skeeters am a-humming" may be fairly authentic dialect of that time and place and I don't hear it as any more offensive than commonly used folk, country, blues idioms such as "I ain't got no" or "he ain't never done nothin'."   

But I agree that it's the sound of the melody and the overall feel of the song that's most important. I don't think that minor changes to the lyrics hurt the song, as long as they sound natural and keep the spirit of the song.