There is one reason The End is Nigh is rated Mature on Switch. It has nothing to do with blood and guts, or sexual content. It all has to do with language. The game is filled with F-bombs. Why not remove/bleep the cursing to reach a wider audience with the game? Edmund McMillen talked about why retaining the cursing is important.
"When I started writing the story of the game it only had a few f-words. Back then I thought [of] possibly bleeping them because it wouldn't lose much, but then late in dev those curses kinda set a tone and I ended up swearing a great deal in the game's dialog, so I saw no point."
Cursing in games isn't anything new. Plenty of games throw cursing around left and right. The thing is, those games usually have their fair share of violence in them as well. The Mature rating is for everything included. The End is Nigh's gameplay would probably earn it an E10+ at most. If you're currently begging your parents to grab the game for you and they won't, perhaps letting them know the details behind the Mature rating could help your cause.