Is this a predicative adjunct?

As Em1 notes in the comments:

She is the subject and is necessary. bore is the verb and part of the predicate and is necessary. It's transitive and requires an direct object: them all. So, all you need is: "She bored them all". Stupid is part of the predicate "to bore (so) stupid" but doesn't add any value.

Using the classifications noted on Wikipedia, "stupid" seems to fit best with "measure":

Measure - Measure adjuncts establish the measure of the action, state, or quality that they modify

• I am completely finished.
• That is mostly true.
• We want to stay in part.

"Stupid" is, more or less, a degree of boredom and isn't a literal state of being stupid. Or, in other words, she didn't cause them to become stupid; she simply bored them greatly.

" she bored them all stupid"

"stupid" modifies the object "them" so I would call it an object complement or if you want "a predicative complement modifying the object". Much longer.