Indeed, not always a more naturalistic, realistic approach to art - which certainly requires more details and accuracy of proportions and so on - is much more complex than a more stylized and abstract style of art. Egyptian painting was famously much more stylized than Graeco-Roman painting, but I'm sure that didn't mean they were less intelligent. More recently the medieval paintings were much less naturalistic and detailed than the Renaissance paintings, even though the very same people, actually just a few generations later, were making them.
Also, some people are just more involved with visual arts, therefore they develop it more than others, and that's all. Some others have other interests, some of which may be simply missed by us because they don't last well (e.g. poetry, music, dance). Art is cultural, contingent, learned and evolved using the same brain capacity. We should not presume a people should always be operating at "maximum capacity". The kind of art made by Aurignacians is well within the grasp of any modern human population, so I don't think they should've been super-smart to be able to make those paintings (which in my opinion aren't better than the Magdalenian ones either).