We have never had a reservation that late but we've stayed at the DLHotel and have walked past the restaurant to go back to our room later at night and got to see what was going on . . . There are definitely characters, and lots of them, regardless of the hour.
I remember one time, in particular, wishing we had made a late dinner reservation instead of a breakfast one because Max, Goofy's nephew, always seemed to be there at dinner! (Our son was a huge Goofy fan and we wished he could have had pictures with Max, too.)
Now, this was a few years back, but I can't imagine it would be any different now. I really doubt that there would be fewer characters just because of the later hour.