To be honest, I've never paid attention to how many times Angela has called Brennan 'Sweetie' but I don't have a problem with it.
That 'Sweetie' is part of who Angela is and causes their sisterhood to stand out within the lab because it lightens the atmosphere and softens Brennan in a way Booth can't quite achieve. On that note, I really wish Booth would call her 'baby' periodically, why was that just in 05x1 'Harbingers in the Fountain'? That moment gave me excited chills and it never happens again. :'(