The fisheye is traditionally used by skateboarding photographers (like me) just because of this, @ic-racer!
The center of the frame gets “pushed away” farther than the borders — that’s why the icicles in the top of your photo look the same size as those in the middle. In a skateboarding photo, using the right framing, the skater looks much higher than (s)he is and obstacles look bigger.
Pretty much every fisheye I have seen has this characteristic and maybe that’s why the corners are not so sharp as the center.
I have the 30mm f/3.5 for my Hasselblad, but I have never tried to assess its sharpness at all. I only use it for skateboarding and customers in this market are not interested in “technicalities”.
Edit: “assess”, not “access”!
My friend Chip Simmons did a ton of high profile advertising including a lot of album covers with that lens in the 80’s and 90’s.