I agree with you. I don't think Lady B is cowed by Sir Thomas, or that he has ever been unpleasant to her. She lets him do her thinking for her, just as she lets Mrs. Norris run her household for her. She's happy to follow his lead. Before her marriage, she probably followed her older sister's lead, so she's used to it.

We all feel Lady B is not the sharpest tool in the shed, and I agree, but in some ways she's very clever - she managed to snag a rich, titled husband, she has all the comforts and conveniences of a large house with plenty of servants without any of the responsibilities. Everyone, from Sir Thomas down to Fanny and the servants, do whatever she wants, and spare her any efforts or inconveniences.