Is there any difference between the sentences?

You were dressed beautifully. (passive voice; the focus is on what she/he wore)

You (had) dressed beautifully. (active voice; the focus is not only on what she/he wore but on the effort (of dressing) itself)

Note that the use of the past perfect suggests some prior time; that is you need some further context to justify its use.

Thanks for your reply. However, I couldn't get "but on the effort (of dressing) itself)". Would you kindly elaborate?

It means that whoever said that sentence thought that she must have put a lot of effort to dress like that.

The difference is in the time depicted. The first sentence is past tense and depicts the state of dress at the time in the past being talked about. For example: You were beautifully dressed at that party.

The second sentence is past perfect and depicts a situation that has already been completed at the time in the past being talked about. For example: You had dressed beautifully for