Seiken Densetsu 3 (along with Rudra no Hidou and others while we're on the subject) uses the SNES's high-resolution functionality (bluh bluh, can't remember if it was one of the enhancement chips or built-in capabilities) to display text. I'm assuming that said high-res mode is not supported by the emulator in question. Too bad there isn't some kind of interpolation method to make up for it.
The english patch's font might be thick enough to be readable at low resolutions, if you want to try that.