A tricky question. I've always treated my characters "seriously", in that I treat them as if they were real people with real histories and motivations and so forth, so even when they're seemingly silly in concept they are always ready to be serious.

I'm thinking probably the one who fits the "silly" moniker best was Puddle Dapplesky. She was a plasmoid paladin in a Spelljammer setting who was dead set on being an elf.

Not physically, obviously - she wasn't an idiot. Indeed, she was proud of her plasmoid physiology and often expressed bafflement at how attached everyone else in the party were to their bones. Bones only hold you back, she insisted. Other organs too, for that matter. She sang a couple of songs over the course of the campaign and one of my favourites was titled "You Don't Need a Brain to be Smart" in which she pointed out all the downsides of having a brain (you can't squirt it through small openings, one good blow to it knocks you out) and that real intelligence came from the heart (metaphorically of course since she didn't have one of those either). She'd often propose plans that required the party to get rid of their bones, sigh when they inevitably refused, and grudgingly propose an alternative that accounted for their inconvenient hinges and stuff.

No, she wanted to be an elf in the sense of being accepted into the Elven Navy as a proper Elven naval officer. She studied the rules and practised them fastidiously. The fact that she was crew on a pirate ship wasn't a problem because early in the campaign she managed to wrangle a letter of marque from the Elven Navy as a reward for helping them out on some issue that slips my mind. So she was a privateer. Entirely different from a pirate, and entirely by the book.

She had decided on this life's goal because a derelict Elven naval ship crashed on the asteroid her plasmoid colony had been living on since time immemorial (it was an extremely dull place, they didn't bother with a calendar since every day was exactl the same). There was a dead elven paladin on board and she claimed her plate mail, literally pouring herself into an Elven mould. The armor had some spiffy magical properties, including a "once a day when making a death save you get an automatic roll of 20". This would have been really handy if she didn't enjoy grappling so much, and as a result would very frequently in mid-combat jump right out of her armor (a bonus action due to her amorphous nature) so that she could get her plasmoid grapple bonus for grappling while naked. I had a separate icon representing her armor I would place on the battle map when that happened so I'd know where the armor was if she got the chance to return to it. Grappling wasn't usually the most optimal thing to do, but she liked showing off her plasmoid prowess. She was also shockingly strong, most opponents were surprised to be physically overpowered by a blob of blue jelly like that.

Later in the campaign she got a cool magic sword that was specially designed for plasmoids. It consisted of hundreds of razor-sharp flakes of steel that floated around in her body until she needed the sword, at which point she could "assemble" it like an extension of her body. Essentially a pseudopod blade, she could take it through one-inch gaps with her. It was a really cool sword. It was also cursed, the sword was possessed by a demon. She knew this when she bought it, it wasn't a secret, but she was quite confident that since she had bought the sword fair and square she possessed it. Legally. The demon kept trying to tempt her to do evil, and she kept reprimanding it and forcing it to fight for good instead. By the end of the campaign it was starting to crack, her resolve to follow the rules was proving stronger than the demon's will.

At the end of the campaign she ended up doing such awesome things for the Elven Navy that the Admirality - with a huge sigh knowing exactly what she would ask for - offered her a boon. She of course immediately requested a commission and was made a genuine captain of the Elven Navy, her life's ambition. They assigned her as captain of that one ship that every navy has where all the rejects and misfits that they can't drum out of the navy for whatever reason get relegated to. But she was happy for the challenge - she would be the elfiest elf on the ship and would whip all those other misfit elves into shape to show them how to be true elves as well.