I think a big part of it is the common blurring of a Lack of Seriousness with Whimsy, which is something that I run into a lot, since I enjoy copious amounts of the latter in my roleplay. It's similar to the misconception that Serious things are necessarily "Dark."
It's not something that can held up as an inexcusable fault of one's understanding, since they're very similar things, but it's very possible for Whimsy and Seriousness to exist simultaneously. Whimsy, by itself, is either a state of constant change, or a state of odd / extravagant / "larger than life" behavior or appearance. The co-existence of Whimsy and Seriousness is actually what makes certain stories so appealing to large audiences; this partnership is what makes us cry at bright and colorful stories like Up, Wall-E, and Toy Story, for instance. It's also what lets us latch onto the brighter parts of darker stories, such as just about everything related to the Shire in the Lord of the Rings trilogy, or the Ewoks versus the Empire in Episode VI. Whimsy can be used as comic relief in a character (such as the bumbling, fussy nature of C-3PO), but it can just as easily be used to convey an oddity or peppiness that serves as a foil to a current mood, without seeming out of place or solely for the sake of comedic relief (such as Martin Freeman's portrayal of Bilbo Baggins, or really, Martin Freeman's portrayal of just about anything). The Joker, for example, is an extremely whimsical character. But you probably wouldn't consider him Funny or Insincere in most settings and under most circumstances.
This combination also allows for some of the most unique aesthetics or styles in storytelling, such as Neil Gaiman's various works (Coraline being a particularly good example), and just about anything by Tim Burton. The often-forgotten movie "9" has both Whimsy and Seriousness simultaneously baked into its setting and style.
Very rarely have I seen non-serious lalafell in roleplay. But I have seen plenty of Whimsical ones, and play one myself. If you look at the distinction between Unseriousness and Whimsy, you might find that most people play lalafell more seriously than you think.
It's not something that can held up as an inexcusable fault of one's understanding, since they're very similar things, but it's very possible for Whimsy and Seriousness to exist simultaneously. Whimsy, by itself, is either a state of constant change, or a state of odd / extravagant / "larger than life" behavior or appearance. The co-existence of Whimsy and Seriousness is actually what makes certain stories so appealing to large audiences; this partnership is what makes us cry at bright and colorful stories like Up, Wall-E, and Toy Story, for instance. It's also what lets us latch onto the brighter parts of darker stories, such as just about everything related to the Shire in the Lord of the Rings trilogy, or the Ewoks versus the Empire in Episode VI. Whimsy can be used as comic relief in a character (such as the bumbling, fussy nature of C-3PO), but it can just as easily be used to convey an oddity or peppiness that serves as a foil to a current mood, without seeming out of place or solely for the sake of comedic relief (such as Martin Freeman's portrayal of Bilbo Baggins, or really, Martin Freeman's portrayal of just about anything). The Joker, for example, is an extremely whimsical character. But you probably wouldn't consider him Funny or Insincere in most settings and under most circumstances.
This combination also allows for some of the most unique aesthetics or styles in storytelling, such as Neil Gaiman's various works (Coraline being a particularly good example), and just about anything by Tim Burton. The often-forgotten movie "9" has both Whimsy and Seriousness simultaneously baked into its setting and style.
Very rarely have I seen non-serious lalafell in roleplay. But I have seen plenty of Whimsical ones, and play one myself. If you look at the distinction between Unseriousness and Whimsy, you might find that most people play lalafell more seriously than you think.