I found her fallen on the ground.
Is perfectly natural, but it does not mean that you saw her fall, it only means that you found her on the ground after she had fallen. To indicate that you saw her fall you might say something like:
- I saw her fall and ran to where she had fallen.
- I watched her fall and then found her where she has fallen.
To indicate that you found her after a fall, a more common phrasing might be one of:
- I found her where she had fallen on the ground.
- I found her on the ground where she had fallen.
These eliminate any ambiguity about just who fell.
I agree with user gotube that:
I found her dressed on the ground.
is at least unusual, not because of the grammatical form, but because of the content. Being dressed is usual for a person outside, so most often one wouold say simply:
I found her on the ground.
If she was not dressed one might say:
- I found her undressed on the groud
? * I found her naked on the ground.
Also the sentence
I found her knocked out on the ground.
is a trifle unusual. The more common way to express this would be:
I found her on the ground, knocked out.
This emphasizes that you found her, and adds "knocked out" as additional detail. But that is a matter of style and emphasis.