My scan clearly stated naruto got serious with sasuke, then naruto used up all chakra even all those ne as well
In context, naruto used up all juice in vote2 and lie flat with sasuke unlike vs kaguya which he had plenty left for vote 2. They are rsm/3 headed dragon/bijuudama/3 tsb/spirit bomb and also 2 arms
Naruto used up above vs sasuke and one arm as well. That is pretty serious just as mangaphaxx stated
Like what Obito supreme jin said, Naruto was serious when fighting Kaguya from start till the end.
Be serious doesnt mean It only when you facing final attack only, when you facing somelike Kaguya that can kill you if you make a tiny mistake, you need to focus and be serious.
In VOTE 2 Naruto got serious when Sasuke ready to lunch his life wiper attack(Indra Arrow) on him and he need to be serious and focus. But before that he never get serious. His Kurama avatar stalemate Sasuke PS like nothing. And he also lower in chakra after numbers of battles. But still managed to fight on par with Sasuke.
But with Kaguya he cant afford direct confrontation like when he face with Sasuke, its to dangerous. Thats why he used clones to fight her. He know its not easy to fight Kaguya and using ninjutsu is useless. He can used Ninjutsu on Sasuke.
He put more effort in battling Kaguya than Sasuke. Not showing Spirit Bomb at Kaguya doesnt mean he not serious. He even throwing 9 TBB FRS on her and its still not killing her. What Spirit Bomb can do to her?
Naruto seriousness on Kaguya is different than Naruto seriousness on Sasuke