Being dark gray, and matte, it probably is less obvious than if it were a painted part. And, ultimately, these cars are built to a price.
I too, have joined the "miniature mudflap crowd." It's very hard to notice, with all the white & black I got goin on... but it's there. (I noticed it the first time I waxed the car.)
This is what it looks like on mine. On both sides. It seems there for some aerodynamic reason. And not something "forgotten" or wrong. It is like that.
It's such a small little thing. 2.5 inches maybe? Just another one of those amazing little "oddities" that I enjoy discovering on the Prius.
It's also on mine - both sides, and judging from the observations of other owners, it is standard and intentional, not a poor fit.
I thought maybe it was for stabilization of those two side panels (where they meet). I have the exact same thing... but not very noticeable on my darker colored car.
I got down on hands and knees to see underside. It appears to be bracket to hold the fender well liner. Looks like your photo. Bracket should have been made beveled to fit flush to the fender, in my opinion. stt
Does the parts diagram give this part a name? It could be for structure support, yet what a bad fit. I'd tend to go with the aero assumption...but what is that air flow deflector doing?
By the way, isn't that "well liner" material a thing of beauty? I was surprised by that. Almost like a material you'd find in a trunk. Back OT... I agree with you... it should have been made to fit flush to the fender. Oh, Toyota last minute sloppiness. At least it's consistently that way on all of our cars!
It's there for a reason. That reason is currently beyond my understanding, but based on my experience spending the last 20+ years as an engineer, it is clear that someone has selected a very specific shape, location, and orientation of this small piece. I've seen lots of accidents, mis-steps, and design errors - this doesn't look like one of those. That said, I still have no idea why it is where it is and shaped like this...
maybe one of the 'prius experts' will jump in with the definitive answer. we should do a contest, and a prize for the winner. i think it keeps the airflow from interrupting the side sensor.