I just had this sentence off my head:
A crown of fire spread through the country consuming everything on its way.
What I mean by empty metaphor is a metaphor that doesn't really have any meaning or even purpose. The same sentence could be replaced with the following:
A circle of fire spread through the country consuming everything on its way.
Also, I don't even think crown would make sense even if the "fire" was spread by an evil king. What do you think?
It isn't exactly empty; a spreading circle of fire might look like a growing crown; with high fire on the perimeter and no fire in the middle. (But ending with "on its way" throws me, it should be "consuming everything in its way").
That said, aside from your example, if your metaphor is obscure and has no obvious application to what is being described, then it breaks the flow of reading because the reader struggles to reconcile this. That would be bad style.
"A crown of fire" is a an evocative statement full of imagery and connotations. Instantly, I can imagine the burning ring of flame, progressing outward and reaching up to the sky. The sentence may not make literal sense but certainly is a good description.
In contrast "a circle of fire" seems somewhat lame. It describes the physical shape of the fire (potentially 'ring' would be better) but it does little to describe the appearance of the flame.
More generally there isn't really such a thing as "bad style" just different style. Whether this is good or not depends on if it fits into the style and structure of the rest of your work. A single empty metaphor in an otherwise literal piece will feel out of place. But if your work is full of this type of description then it will be fine.
Similes and metaphors are types of analogy. It's still a metaphor because of the sentence structure (it's not a simile).
When the comparison is really extreme it's called an analogy. The brain has to "reach out" to synaptic connections find the cross-reference how these things are alike, and eventually offers up shapes and colors, like your fire-crown.
It's a good instinct. Keep listening!
It is not "empty", it is loaded with vivid imagery, but also other (surreptitious?) cross-referenced associations like concentration of power, danger, and eminent domain. My brain was delighted. Thank you!
Compare to this example from the analogy link:
Night Clouds (By Amy Lowell)
The white mares of the moon rush along the sky
Beating their golden hoofs upon the glass Heavens.
Which plays with associative imagery and motion, but makes your head hurt if you try to picture it too literally (horses, upside-down, tapping on a class ceiling with their feet).
You are right to be wary. An empty metaphor is a missed opportunity, and missing your opportunities is bad style. Please note that this doesn't mean this is a "bad" metaphor. It means that it's a metaphor that isn't (currently) working. In the right context, it could be a great metaphor. In fact, it might be leading you in a productive direction, one you just need to expand upon.
I torch the forests with a sword of flame, and top the trees with a crown of fire.
Now this metaphor is doing work. It's telling you things about the narrator (the evil king you suggested). Here's a different example:
The regal forests were robed in green, but crowned with fire.
In this case, it's the forest itself that's personified as royal. These usages are easy to understand, because there's some context letting you know what connotations of crown are important. In your original example, the reader has to work hard to guess what you mean. A crown is a circle, it has jagged points, it goes on the top or the head, it signifies a monarch. So which of those is applicable here? If you're just going with the simple, geometric answer, you're right, you're not gaining anything over the more direct "circle." In fact, you've made your job harder because crowns don't expand, and a circle of fire does.
