Help me understand this part from Emma

I am reading Chapter 14 from Part 2. It's the chapter where Mrs. Elton supposedly reveals herself as being 'vulgar' but honestly, I'm not getting what she's doing so wrong? For example this part:

"We have been calling at Randalls," said she, "and found them both at home; and very pleasant people they seem to be. I like them extremely. Mrs. Weston seems an excellent creature--quite a first-rate favorite with me already, I assure you. And she appears so truly good--there is something so motherly and kind-hearted about her, that it wins one directly. She was your governess, I think?" Emma was almost too much astonished to answer.

I'm not understanding how what Mrs. Elton said was so astonishing or vulgar? It sounds like she was just saying a nice thing about someone and asking a reasonable question.