I think a lot of it is just Dazai’s tone in general, but one big thing I notice about the passage (having read it out of curiosity if I could answer your question, so admittedly I was also looking for it), is that the whole of the imagery he paints about the ocean and the flowers seems to be a lead-up to subtly and bitingly cast aspersions on this school and its students’ futures.
Like, he weds the image of these cherry trees shedding beautiful blossoms that are then being battered directly back into the shore, directly with this image of a regional school he didn’t bother studying for but still “safely” got in - and then as the punchline the school proudly spackles those same cherry blossoms (that are smashed right back into the shore as soon as soon as they leave the tree) on all their insignias and all their students. Implying that they’re never going anywhere else either.
That’s my take anyway!
I think it’s less a case of text and more a case of subtext and what descriptive imagery is being used for, rather than the imagery itself. (but I could of course be off-base, and someone could have a completely different interpretation)