Should I be ashamed of myself for taking 3.5 years to graduate college instead of 3 years flat if my parents were paying for it?

No, as long as you are doing what you honestly feel is best for your development/future in a responsible way. All parents just want what is best for their kids, and determining that isn’t always easy or clear. Best and fastest/cheapest aren’t the same. Would they like to save some $? Sure, but probably not at the cost of burn out or sub-par grades that might impact your future.

Now, if you just want to have no classes on Fridays so you can party more, that might be different (but honestly, some parents might even be cool with that as they might view it as “work/life balance”).