Happy New Yeare to everyone!
In my opinion, the ideal for out of focus foliage backgrounds is that there is no definition in the elements behind the subject. In other words, there are no defined branches, leaves, trunks of trees or flowers. In the picture above of an eyelash viper in Costa Rica, notice how the background is mottled in terms of light and dark, but there is no extreme contrast and none of the plants and trees in the background have definition. This means that the greenery is complementary and non-distracting.
If you can't get this kind of background with the original digital capture, then you can replace a distracting background with something similar to what you see here using Photoshop. I have a folder of two dozen out of focus foliage backgrounds in my photo collection for this purpose.