What shader? If you hadn't noticed, then it did its job well.
While SNES games have always been displayed in a 4:3 ratio, the console's actual internal resolution has an aspect ratio of 8:7, meaning the image would get stretched. Some games were designed with this in mind, Yoshi's Island clearly wasn't (e.g. the moon isn't a circle). Still, 4:3
is the "correct" aspect ratio! Problem is, forcing aspect correction in higan (or any emulator) inevitably leads to rounding errors. It's not that noticeable in this game with so many complex shapes and few straight lines. But it can get ugly when the screen is scrolling at a constant pace. Look at Green Yoshi in this example.
His eyes should have the same width, but the right one appears thinner. Entire pixel columns are affected by this, it's everywhere. Usually this gets fixed by applying filters/shaders that smoothen the image, or add scanlines and whatnot, to emulate a low-res CRT screen. Higan offers another option, which I'm sure is old hat, but I haven't seen it in other emulators before: AANN ("anti-alias nearest neighbor").
It's magic! Suddenly every pixel looks the same size. How?
A closer inspection exposes the trick. Nifty.
I've also tried other shaders like the usual scanlines or even NTSC. They can look good on other games, but for this one AANN works best imo.