Why is Kuja such a noob?

[Xurts]

The problem is... You discarded all of Kuja's character(his other tantrum and pissed off moments) and all the good reasons that made him have that anger tantrum(the fact he worked his entire life for that moment, only to discover that everything he did was useless, because he was going to die anyway), just to give support to your crazy theory.
Yes, a theory. You made up actions or events that happened, by stating that Brahne was influencing him. Something never stated or hinted during the story.

The "other tantrums" you mention, even though I don't remember any of significance other than his Trance, I'll give you the benefit of the doubt, composed only a small part of his character. Every other moment we see Kuja he's always confident, collected, and calm, even during stressful situations such as the battles at Alexandria and the Iifa Tree. His last moments were nearly, if not the only times we see Kuja express such strong emotions like anger.

Kuja's actions at the end of the game are understandable. For the entire duration of his life he assumed he was immortal, and he devised a somewhat elaborate plan to overthrow Garland. It eventually worked, and he gained immense power through Trance. But at the moment of Kuja's greatest victory, Garland tells him that he is indeed a mortal. His burst of rage after he realized that all of his accomplishments would soon be worthless is perfectly understandable. Kuja didn't have any realistic options left to him at that point, so he chose destruction. It's not exactly honorable, but it's understandable considering his situation.

He was in front of the Original Crystal, the source of everything in the universe. He could have tried to use it's power to stop his dead, instead of destroying it. But no, he didn't even try.
That's the only moment in the story where Kuja was stupid IMO.

What was Kuja supposed to do there? Give the crystal a great big hug and hope it would grant him immortality? Not likely.

Brahne didn't influence Kuja's actions. He mentioned that she had a particularly strong and wretched soul, and I'm sure it enhanced his Trance more than regular souls, but I doubt it had any influence over his actions. He had control over his Trance, Brahne's soul just enhanced its strength.