sickly sweet
Nauseatingly cheerful and inappropriately informal way of speaking, including excruciating use of high-pitched giggling interlaced with overly familiar colloquialisms against a backdrop of feigned empathy and mis-stated cliches.
Like the sickly sweet airhead who answers the phone at my office. Instead of answering the phone like a professional she starts out with "HI HUNNEEEEE!!" to someone she's never met then finishes up giggling "HEE HEE HEE HEE, I'll give him the message SWEETIE......OK, you poor honey baby... six of one is worth two in the bush, buh-bye hon!!!