According to the article Verbless Sentences by Bill Ball on the website of The Queen's English Society, your example of a verbless sentence is OK. First, what is a verbless sentence, and is your example verbless:
Although there have always been verbless sentences in English, many
grammarians of old insisted that a sentence had to contain at least
one 'finite' verb. Examples of finite verbs are 'is', as in 'The
weather is fine', and 'plays', as in 'He plays tennis'. The word
'finite' broadly means 'having a subject'. In the above examples, the
subjects of the verbs are 'The weather' and 'He'.
The author then gives several examples of verbless sentences, of which A wonderful achievement. could be an ironic commentary on Peter Goodman's Robin Hood, reversed.
Next the author cautions about the use of verbless sentences:
Indeed, there are still some people today who would condemn verbless
sentences, even though they are and always have been acceptable
It has to be admitted, however, that verbless sentences should never
be used in formal writing (legal documents and the like) or by
schoolchildren or students in their school or college work. It is
often said that it is acceptable for established writers to commit
occasional grammatical errors, because they have learnt all the rules
and therefore have the right to modify them to suit their purpose if
they so wish. That may be so, but there is one 'rule' that says that
verbless sentences should be used sparingly and with good taste. Any
writers who choose to ignore this rule (for whatever reason) do so at
their own risk.
In the example the OP gave, the verblessness of the concluding sentence added emphasis. This is Robin Hood in reverse would have had a much lower POW!- factor.