Theoretically for a couple of reasons. The Scrabster-Stromness and Pentland ferries boats run several times a day but the Aberdeen one is only a few times a week, and the journey time from the Central belt can be less. The main issue is that getting there is a lot more awkward and complicated, taking a couple of trains and a boat where the timings aren't necessarily in sync. A direct boat train or sleeper would take the hassle out of it and make the route a lot more attractive. Caithness could also do with improved links to the rest of the country.
But, for various reasons it's not gonna work.
You can increase the frequency of the Aberdeen ferry by buying another boat. That boat can also be used on other services.
Sleeper carriages don't have any other use, & there wouldn't be other routes for them.
Direct boat train or sleeper from where (?) would be less hassle?
It's ~6.5hrs from Stirling to Thurso.
So yes, a sleeper train getting into Thurso early morning before breakfast would time ok from Edinburgh or Glasgow, but it would be sleeper only. The timings don't work for doing dinner or breakfast, which i'm sure are part of the attraction of a sleeper service.
Then the train does what?
Sits at Thurso for 12hours+ to then do a return to Edinburgh? Which means that with 1 train/set it's an every other night frequency, so not all that different to the Aberdeen ferry.
The 'fix' for train <> ferry is increased frequency & shorter travel times.
4 trains a day & taking 4 hours is the problem.
A sleeper service doesn't fix that.