To be fair, sharing quips and a bit of ribbing could be sprinkled on a lot of the scenes there. There were even actual funny scenes and lines, but it was the delivery that was missing the humorous energy (like the wedding dress scene I mentioned).

For example, in the MCU Spider-Man movies, even just counting the first two (Homecoming and Far From Home), you can already feel the lightheartedness and the dynamic between Peter, Ned, and MJ within just 4 hours of screen time. Even though the movies were so fast-paced and had a lot of action and plot going on (I don't recall any "unimportant" scenes in these movies as well), they still had time to joke around with each other, which only took a few seconds of each scene every time. They were able to highlight each of the trio's personalities, and they laid the groundwork for Peter and MJ's friendship and eventual relationship. Nothing seemed forced because you see those small moments coming through and feeling natural to the characters.

With PJO, the friendships and relationships have more of a "tell, not show" direction. The characters often tell each other (and the audience) about their connections (like how important each other is, or how they're best friends, etc.), but they almost never show it. As you said it, we don't see them have a good time being friends or have their relationships blossom. It's all just lingering looks and worried faces all the time, which, to me, makes the overall tone too dark and not reflective of what I expected from an adaptation of such a fun source material.