If you talk to Isabelle, the island eval should help tell you if there's too few/too many of things like trees, flowers, etc.
Sometimes it's something having a negative impact, like 'clutter'. If you have dropped items, materials, tickets, lying around, that can bring down your score.
I think weeds don't matter unless you just have way too many of them. Do you have all of the island shops built? 10 villagers?
If you have all of that, maybe the arrangement of the outdoor furniture? I had small furniture additions spread out on the island instead of in just a few spots.