People with degrees are more likely to have developed good study methods than Joe Bloggs. This will put them at an initial advantage to those without degrees where studying for the ATPL theory exams are concerned.
However, as has been said, there is no substitute for perseverance, and plenty of people without degrees have loads of that. Also don't discount the factor of people with degrees being more blase about the amount of study required, than a keen non-graduate.
cheers!
foggy.