"Goyslop" and "sloppy" are both derived from "slop". "Goyslop" meaning "slop of the goy variety" (equivalently to "AI slop"), and "sloppy" meaning "having the qualities of slop".

I was already aware of the term "goyslop" but I see it as a parallel development rather than the origin. I heard daytime TV programming being referred to as slop over twenty years ago. I don't think there are any antisemitic undertones for the word outwith the "goyslop" coinage.