I think you are right. Now that you mention it, your interpretation of Mary's "not born to do nothing" line as a dig at Edmund's choice of profession seems perfectly obvious, but it hadn't occurred to me before.

Taking the thought further, what Mary says in this passage could have multiple meanings, all at the same time. Mary isn't an analytical person and her speech could reflect that. She just opens her mouth and the stuff pours out, so her words could well have further layers of meaning as well as the surface one. Translated into plain speech she could be saying:

"Don't be a clergyman Edmund - they do nothing and are an idle waste of space."

And: "You aren't doing enough to win me. You have to do more, stake your all and give up the clergy if you want to marry me."

And: You may not be willing to make the effort to save our relationship but I am. I'll stake my all and do everything I can think of to make you change your mind, even if I risk losing you."

Of course Mary would never think of what she says in this way. If asked to deconstruct what she had just said, she would just shrug her shoulders and brush the question off with a witty "do not attack me with your watch" type of line.

I totally agree with Barb JA about Mary's lack of respect for Edmund and what a bad sign it is for their future. I think Mary's real attitude is harsher than ridicule though. Ridicule is what Mary has used against Edmund's choice of occupation ever since she first heard of it, but I think this conceals her true feelings. Ridicule is a prettied-up version - she can still sound feminine, light-hearted, amusing. We see the truth in ch.29 when she is looking back at the the night of the ball, her last opportunity to change Edmund's mind before he goes to be ordained:

She was afraid she had used some strong, some contemptuous expressions in speaking of the clergy, and that should not have been. It was ill–bred; it was wrong. She wished such words unsaid with all her heart.

Contempt is a very strong word indeed. I believe contempt is what Mary has been feeling all along, - contempt for the clergy as an occupation, and by extension contempt for the man who undertakes it. It looks like a remarkably bad omen to me!