This study evaluated the efficacy of a novel group intervention intended to increase the SWB of early adolescents who are less than delighted with their lives.
Is it a typo in the text putting than before delighted or is a special combination?
This looks an example of understatement used for emphasis, I suspect it's peculiarly British although it may be used elsewhere.
Delighted, as I'm sure you know, means very pleased.
Less than delighted literally means simply not delighted. In theory, a person who is "less than delighted" might be anything from very cross indeed up to quite pleased. In practice it will generally mean something closer to the very cross indeed end of this range.
Understatement is an arrow in the rhetor's quiver. Some folks use it more than occasionally.
OK, now why did I say "more than occasionally" instead of "frequently"? Because I like using the rhetorical figure of understatement.
A variation on understatement is litotes (lie' toe tease), such as "There was no small stir among the protesters," instead of "There was a huge stir among the protesters."
Why do folks use understatement? I imagine simply to introduce variety into the way they speak and write.
less than delighted = not delighted
Please look up the phrase "less than" under less in The Free Dictionary. Accordingly, it means "not at all". He had less than favourable view of the matter.
Please also refer to Longman that says the phrase is used to mean " not at all". He is less than enthusiastic about this idea.
However, according to the definition given by Cambridge, you can say that the phrase less than delighted means "not delighted".
So less than delighted means not at all delighted or not delighted.