I was just in Bar Harbor this week. There wasn't anything I saw there that would be considered "luxury" by most (or all) in this forum, but there were some decent places. We stayed at the Ullikana Inn:
https://ullikana.com/
I would recommend it for the great location, and the beds were very good (they say they just upgraded all the beds at ~$3000 each). My wife liked the towels and the Molton Brown toiletries. The location was excellent, a minute or two walk from downtown, but just far enough off the main street to be quiet. Dedicated parking, which is great because the parking in Bar Harbor can be crazy when it is crowded.
We walked by another larger place that looked decent, the Bar Harbor Inn. It is on the waterfront, and had some balcony rooms facing the harbor. The Ullikana was a small B&B vibe, and the Bar Harbor Inn is much larger.
If you are planning on exploring the area and want another recommendation, I HIGHLY recommend the Inn at Sunrise Point. It is in the Camden/Lincolnville area, south of Bar Harbor. It is right on the water, with beautiful views and amazing sunrise view (as you would expect by the name). We had a cottage facing the water, and it was probably the best B&B we have been to.
https://www.sunrisepoint.com/
The Homer Cottage seemed to be in the best position and most private.
EDIT: I also should have mentioned that for the spring you should check the schedule in advance. Many of the places we saw in the area are closed until April or May.