I don't think the Mirkwood Elves are more serious, it's just that we see them in more serious situations (catching intruders as opposed to receiving guests, plus being in an army and fighting a battle and what not) but before that what we see is a buncha Mirkwood Elves throwing a party in the forest and one guard drinking himself to sleep, as you allude to

Tolkien's elves are more attuned to nature and they care deeply about trees and stuff, but the latter at least would also apply to a lesser or greater extent to most of the Free Folk. I wouldn't be surprised if the tree-hugging elf archetype came in no small part from the later association of LotR with hippies