I’m not sure about the right choice of English words and adjectives but avoiding lookalikes ones might help to better understand the sentence.

1.

Curiosity is one of the steadier traits

2a.

[Curiosity is one of the] clearer traits of a sharp intelligence

2b.

Curiosity is one of the steadier and clearer traits of a sharp intelligence.

By the way, Samuel Johnson was British so his quote was written in English and reads:

Curiosity is one of the permanent and certain characteristics of a vigorous intellect.

Most was “unlost” in translation…

To better stick with the English, that might have been translated:

La curiosité est l’une des caractéristiques permanentes et certaines d’une intelligence vive.

"La curiosité est une des caractéristiques les plus permanentes."

I’m not sure what this means.

• Does it mean that out of all possible characteristics, curiosity is the most permanent?

• Or does it mean that out of the set of “the most permanent characteristics”, that curiosity is one of characteristics in this
  set?

It means “curiosity is one of the most permanent characteristics”, so definitely more along the lines of #2 than #1.

If I translate this outside of its context, I have “and certain of a
sharp intelligence”

Despite their proximity in the sentence, “of a sharp intelligence” is not a complement of “certain”, it is a complement of “caractéristiques”, hence your confusion.

You could rephrase the original sentence as:

Parmi les caractéristiques d’une intelligence vive, la curiosité est une des plus permanentes et certaines.