Can thoughtless serve as an antonym for pensive?

I can see why the Word thoughtless might seem to be an antonym for pensive, since a pensive person thinks a lot and a thoughtless person doesn't, however, thoughtlessness is really about irresponsibility, it is not really about thought in general. The opposite of thoughtless is thoughtful or responsible, not pensive. The opposite of pensive would be spontaneous, or glib.