They were of course all intending to be surprised; but their astonishment was beyond their expectation, and on the part of Mrs. Bennet and Kitty, though she was perfectly unknown to them, even inferior to what Elizabeth felt. (Chapter 56)

I think the early visit was unusual but I think the surprise and astonishment is primarily due to the unknown equipage and livery which no one recognizes.

I think Lady Catherine would be rude during her visit to Longbourn at any time of the day considering she is angry and the mission she was on—to stop Lizzy from marrying her nephew. IMO Lady Catherine was showing her distain by showing up at an impolite hour—a kind of double-whammy, rude by showing up too early and rude in her behavior.

I think the time of the visit is stated in the text to illustrate Lady Catherine’s distain for the family but neither Lizzy nor Mrs. Bennet seems to take offense at the early hour. Mrs. Bennet focuses on the compliment of the visit while Lizzy recognizes Lady Catherine is more than usually ungracious and puzzled by the reason for the visit.