For if knowledge is power, then a god am I. --- The Riddler, Batman Forever, 1995
Please note that "a god" is a third-person noun phrase. It does not agree with the verb form "am".
Nevertheless, it is grammatically correct and perfectly sensible. This is an inversion. Normal word order for the clause is "I am a god." The verb "am" agrees with its subject "I", and the third-person complement doesn't need to agree with anything. The inversion simply lends emphasis to the complement. The phrasing "a god am I" reads like "I am a god!"
The same thing happens in your example: Those little oranges are a scary thing. The verb "are" agrees with the subject "oranges". The complement "a scary thing" doesn't agree with anything. This subject/verb agreement survives the inversion: A scary thing are those little oranges.
Was that over the top?
That is the Riddler's next line in the scene. Here, "over the top" means excessively dramatic or excessively emphatic. That is an accurate description of his prior line. He just delivered that emphatic word order in a strongly emphatic tone -- loud, rough, deep, and slow.
The beauty of this example is that the source material itself immediately indicates the reason for the Riddler's use of the inversion. He clearly intended the phrasing "a god am I" to be as emphatic, as poetic and as dramatic as possible. He intends to be over the top. He could have said "I am a god", but that ordinary statement lacks the additional impact that the inversion grants.
The emphasis in your example is not over the top. It's a natural emphasis that many native speakers understand and use instinctively, without even noticing or recognizing the grammatical mechanism involved.
In other words, it's grammatically correct, perfectly sensible, and utterly common.