Hi Tobias,
My feeling is that what you did on this is acceptable.
The decision on 'how much is too much' can only be determined on a case-by-case basis.
Factors determining the outcome might be;
intended image usage
composition
subject species
subject conditions
what has been done
risks of causing harm/distress by trying to get a clearer shot
editor's intent
Taking a more extreme example ...
We may (as photographers) often shoot animals at zoos through glass, mesh or bars and we'll use various techniques to overcome these at time of shooting and quite possibly also in Post Processing, to give a clearer view of the animal. As long as we don't try to pass these off as 'shot in the wild', most (especially here) would agree these actions are acceptable.
That said, someone with a strong views on the incarceration of animals for gratuitous public display and profiteering would probably say that is going too far and if they shot the animal themselves, may take an opposite course of action to highlight their captivity.
In certain circumstances, e.g. seeing distressed caged animals for sale in markets, almost anyone that might do the former normally will probably do the latter.
I like the darkened highlights in Jeremy's example.