Fair point, but then again, what's the difference between intentionally being passive cause the meeting isn't relevant, and actually wanting to pay attention, and finding out that you drift off without noticing, i'm more interested in the second case.
If the meeting is boring to me, it's because the topic of the meeting isn't very useful or relevant to my role. I stop paying attention and think about whatever thing is my current priority.
If you're obviously invited only as a "seat filler" (and why else would the meeting be boring?), I see nothing wrong with being passive.