Mary Crawford has high connections and 20000 pounds, and her sister thinks (rightly) that she may well hope to marry a future baronet. Mrs Elton is alright financially (10000 pounds), but she comes from a background of (arguably disreputable) trade.

The problem with her "protection" of Jane Fairfax is that Jane was a gentleman's daughter and therefore she shouldn't be treated as an inferior but rather as an unfortunate equal. Helping her to find a position would be a good thing in itself, but under conditions: that Mrs Elton would be discreet about it, and actually allow Jane to say the word "no". Mrs Elton doesn't allow Jane to say no, as if she's not able to judge for herself. And she makes so much fuss about helping her (as opposed to doing it quietly) that one thinks her motive is vanity rather than a real desire to be helpful. That ridiculous public scolding about Jane's going to the post office during rain! Not to mention the familiarity of calling her "Jane".