Do finishes matter more than decisions? I don’t know. What I do know is that when you compare champions and top 5 fighters against their weight class average, the champions and top 5 fighters usually have higher finishing percentages. In fact only welterweight and middleweight have top 5 fighters with lower averages.
There are two ways to look at this. Either having a high finish rate helps in becoming a great fighter, perhaps by allowing a fighter to avoid more damage that would occur later in the fight as he climbs the ladder towards a title shot. Or having the ability to consistently finish fights is an indicator of a higher talent than the average fighter. The other way to look at this is that finishing fights gives you a promotional boost in which the UFC jumps you higher up the rankings each fight than they otherwise would if the fight went to decision. Therefore making it easier for fighters with high finish rates to become champions. The answer is likely a combination of both. Finishes are both representative of the highest ability of fighting and also a promotional tool to market a fighter as a title contender.
It is also not surprising that welterweight and middleweight are outliers here. Dominant champs are boring. Always have been and always will be. Their highly technical skills combined with extreme risk aversion allows them to reign for extended amounts of time as they avoid most of the remotely exciting altercations until they are guaranteed a 100% successful outcome in engaging the other fighter.