It is a metaphor. So without context it is not clear.
In context of idiot it can mean idiot, in this case there are lots of words with very small differences.
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I give some variants:
Birne can be a metaphor for Kopf insome areas in coll. language. In this case "Du bist eine Hohlbirne" means "Hohlkopf" - "in your head is nothing but a hole."
This is basically "you are an idiot."
This might be transfered to the person as metaphor.
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Hohlbirne | Übersetzung Englisch-Deutsch gives:
Edit: crossposted with Demiurg.
PS: I searched whether it is used in a technical sense like "birne=lamp" and "Hohlbirne" is a kind of housing, but it is not. I did not find it.
And I did not find that it is anywhere used for the fruit. It is always used for a person or the person's head in an extremely pejorative sense.