Kind of depends on what kind of pond your talking about. Stagnated ponds that are covered in algea and nasty stuff are not good to use for your plants. You can kill your plants and get diseases with bad stagnated water. You want to make sure and check the ph of the pond water and make sure that it is of the proper ph. Also small roadside ponds if near large roadways can be contaminated with oil and gas runoff from vehicles. In either of those two cases tap water would be a better way to go. But if the waters ph checks out and it is somewhat clear water and fed by a spring or creek you will do fine. I use a small creek fed swamp pond for my outdoor girls and it works great.
If the pond is nice and clean and filter then you can use pond water otherwise tap water is good. Its completely depend on your plants and pond. Some plants need much minerals and nature touch then pond water is best.