A good score depends on the situation. For a horse going steadily in prelim/novice, I'd be expecting scores in the 60's to 70's. Generally, as the level increases, the percentage considered a good score will decrease a little as it is much harder to score highly when you are completing harder movements. However as the horse and rider gain more experience at the level, the percentage should begin to climb again if training is going right.
For an inexperienced horse and rider at an official competion, in the 50's I would consider a good score. You can't expect to go out and get a 60% if the both of you aren't tuned in enough yet, but you can only get better.
Then you have pony club and associate scoring. They seem to be extremely generous, when I was in pony club as a littley, my friend had an old grey pony, full of cancer that went around like a giraffe on roller skates. She got 80% in a PC prelim test. This seems to be the norm with pony club/riding club/and some associate competitions, the judges feel they should be generous to make the rider happy.