Kirby fanfiction doesn't get the same massive traffic as giant fandoms, but there's a persistent little niche on Wattpad that's actually pretty active. The most popular ones right now seem to fall into two camps: the 'character x reader' stories and the surprisingly deep 'lore expansion' ones.
If you search the tag, you'll find a bunch of stories where Kirby or one of the villains like Meta Knight or even King Dedede is the love interest. They're usually short, fluffy things, but there's a consistent audience for them. The other trend I've noticed is authors taking the implied worldbuilding from games like 'Kirby and the Forgotten Land' and running with it, crafting these epic post-apocalyptic adventures. There's one called 'Shattered Stars' that's been getting a lot of reads lately; it treats Meta Knight's backstory with a seriousness the games only hint at.
Honestly, the charm is in the weird specificity. You won't find thousands of fics, but the ones that are there often have a lot of heart, written by people who genuinely adore the source material's strange mix of cute and cosmic horror.
My current bookmark is a crossover with 'Sonic the Hedgehog' that shouldn't work but somehow does, just because the tones are similarly bright and fast-paced.