Well, let's do the math. Convert those color values to the Hue-Saturation-Value model. (Enter it as Hex(D19FE4) and so on.)

283°, 30%, 89% - Twilight
334°, 26%, 96% - Pinkie
34°, 61%, 99% - Applejack
55°, 31%, 99% - Fluttershy
199°, 36%, 97% - Rainbow
204°, 2%, 95% - Rarity

Here's what we get. The Hue is all over the place, expectedly so. The Saturation levels are indeed intermediate, with AJ going the highest. The other common thread is a very high Value as well. What out of that are we really perceiving? Let's try some examples.

Start with Twilight and raise the Saturation: HSV(290°, 77%, 87%) is the color of Berry Punch. Berry is arguably pastel, she's a more electric and vivid purple/magenta, but could fit the definition.

Start with Twilight and reduce the Value: HSV(283°, 30%, 30%) is a dull purple that's hard to describe, it's not really any pony color, it's like a twilight (as in dusk sky) color. Probably wouldn't call that pastel either.

Do both, high Saturation and low Value: HSV(283°, 80%, 30%) is an electric but dark purple (think like Prince's purple guitar), probably wouldn't call that pastel either.

So I conclude that what we perceive as pastel is mostly high Value, with some influence from low-to-intermediate Saturation. An interesting edge case is Luna, HSV(233°, 56%, 48%). That might be right on the borderline for both the upper limit of Saturation and lower limit of Value to be considered pastel.

(This is all in HSV color space. HSV and HSL are not the same thing and HSL is a more useful space to describe human perception: https://en.wikipedia.org/wiki/HSL_and_HSV )