Is it acceptable
The word "acceptable" is very fuzzy. Each person implies something different, based on his cultural background, unique experience and the set of values. Therefore it's impossible to answer this question.
If we narrow it down to:
Is it legal?
Definite "no" in most jurisdictions.
Is it fair towards the author?
Again, no, if the author wants to keep his work private you must respect that.
There is a policy of "fair use" when you can use parts of the copyrighted work in a certain context but that implies you take the public output of the work, as a matter of speech. Decompilation, or in other words (reverse engineering, legally speaking) is an entirely different activity which has nothing to do with fair use. Here you're trying to restore the intentionally hidden work from what was made public. If they used obfuscation then your decompilation is a fact of intentional breaking of protection mechanisms. And it is punished by law.