Well To Make A Long Story Short, My Grandsons Mother

Well to make a long story short, my grandsons mother is not a very good one, she worries about everything else but her own children... I went to a well visit with her with my 2 yr. old grandson and the dr said he is under the third percentile of his weight, I am concerned because she is lazy and don't feed him right. Is being under the third percentile a bad thing? I don't really understand the percentile thing.