I think "the normal one" usually referred to the fact that he didn't particularly excel at anything. Harry was the Boy Who Lived and Hermione was top of the school, but Ron was just an average teenager; ok at most things, bad at some and better at others, a firm friend with a good sense of humour but with the usual teenage social pitfalls. I was in HP fandom when the books were coming out and most people seemed to over-identify/project/wish fulfill on either Harry or Hermione, but Ron was more like the regular friend in the background of your daydreams rather than the character you actually pretend you are.