Does this sentence seem weird?

I disagree that it's awkward at all. To add "that had been" to the sentence would be to weaken it. Golding employs here a rhetorical device known as ellipsis, the deliberate "omission of expected words"; and rhetorical impact trumps grammatical nicety any ol' day of the week.

Here's what Ward Farnsworth (quoted above) says about ellipsis in his book Classical English Rhetoric:

An ellipsis involves the audience in an utterance; the reader or listener fills in the missing language, consciously or not.

Whatever your feeling about the sentence, you noticed it. It got under your skin and stayed with you. That, my friend, is good writing.

A scar smashed into a jungle ought to be pretty damn jarring, don't you think? And if it took you more than a few milliseconds to recover and parse the sentence correctly, I would be very surprised.¹

¹Edit to promote material from my comment to the answer proper.

It's a little awkward but grammatically acceptable. The point of confusion is that smashed in that sentence is the past participle, but the sentence structure invites the reader to initially read it as the past simple (used in the narrative mode indicating present action in the storyline).

It would have been clearer, if presumably less satisfying to Mr. Golding and/or his editor, if it had been written:

All round him the long scar that had been smashed into the jungle was a bath of heat.