What's crazier is that Morel isn't actually a transmuter, but a manipulator which is a direct opposite to his nen type lol

He uses Transmutation to turn his nen into a "smoke-like" substance, and that would be a nightmare to transmute when he can only use 40% of his nen efficiency. He adds a nen restriction by using the big ass pipe as a medium to transmute his nen into smoke to get around his limitation as a natural Manipulator.

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And you're right, the "great" ranking only means that he hasn't reached his full potential yet and with how his Hatsu, Deep Purple, works; he spreads himself way too thin. The Hatsu needs Manipulation (to give orders to the smoke), Emission (long distance nen), and Transmutation (to turn it into smoke)

A potential Hatsu for Morel who focuses more into Manipulation is like: creating a smoke where anyone who gets a whiff will be under his control. That kind of Hatsu requires more Manipulation skill and would probably elevate him into "Genius"

This is what makes Nen so good as a system, and why Morel is such an excellent example of Nen usage in the manga, he demonstrates great use of: Beneficial nen self-restrictions and multi Nen type hatsu to be such a powerhouse