Not entirely. A rather large amount of Linux's mess stems from the fact that it was a hobbyist project in its foundational years. It was never clean or well designed, at any point in its life. Go look at Linux 1.2.0 vs FreeBSD 2.0

Even when Linux began to get traction, it had already developed an ingrained culture that didn't particularly care about "nice" code or architectural solutions. The BSDs inherited their culture where such things were prioritized. You're right that things get messier as they get larger, but the gap between the two is much, much larger than can possibly be accounted for. Things like Linux not respecting NICE values have very little to do with surface-level problems like stylistic inconsistencies in the source code.