Re: Adjective or past participle
I would take it as a past participle used adjectivally. It is slightly defective, though, as it requires a qualifying adverb, e.g.
1. A well dressed man
2. He was badly dressed
So you might take the whole phrase (adverb + dressed) as the true unit.
It differs from another sense of "dressed":
3. Are you dressed?
"Dressed" in #3 refers to the state of wearing clothes; whereas "dressed" in #1 and #2 refers to the style in which those clothes are worn.
All the best,
Not a professional ESL teacher.