Turn down service includes a mattress pad and sheets, so yes, it might be hard to make those from scratch.
The mattress pad is provided upon boarding and convenient to apply whenever one chooses. Are there sheets I'm not aware of? Sorry, but put me in the camp that thinks requesting a special turn down service is pretentious if not downright awkward in mid flight.
On a side note, as one who tosses and turns in sleep, I find the mattress pad to be cumbersome, ending up in a tangled mess with seatbelt and duvet.