Yes, I think that Al Qaeda IS that stupid.
They like blowing people up. It is their vocation. It is their reason for existence. If they can set up another massacre in Spain, they will do so. After all, there is still the expulsion of the Muslims in 1492 to get revenge for. Plus, Spain must be further punished for going into Iraq with the USA in the first place. Plus, Spain has cooperated with the anti-fanatical government of Morroco in its attempts to weed out terrorists. There are many reasons to attack Spain again and again, and Al Qaeda will do so if it can.
Rationality is not Al Qaeda's long suit.