It’s not sinful, but I very much dislike it, and I much prefer the practice of the Jews and the Muslims, to bury promptly in a manner that actually ensures quicker decomposition. We are meant to return to the elements.
This said, I have had to agree to a prepaid funeral and burial arrangement which incorporates modern American practices. My family has very strong sensibilities about this sort of thing (even though, when pressed, they will concede that embalming only delays decomposition) and it was a “package deal” of sorts. If I outlive them, I am seriously considering selling my interest in it, and arranging for “green burial” with no embalming and only a simple, biodegradable container or even just a shroud.
Incidentally, bodies can be preserved for some time by refrigeration, so unless there is some loathsome disease or transport situation, embalming is never necessary. (For obvious reasons, strong embalming is also needed for medical cadavers.)