It would have been edgy if they’d had him urinate on a quran instead.
Spitting on Jesus in Season 3 makes Frank look even more reprehensible since he actually pretended to be a steadfast Christian in an earlier episode, even giving a fire and brimstone sermon in a church (which of course was an election ploy).
I have to admit, it has been kind of fun watching the Underwoods unravel after all their scheming and criminal behavior, including murder.