That and, while this might be a stretch, Big Mom’s silhouette in the magma looks to be a top-down perspective - perfectly round with no visible legs, the points of her hair encircling most of the form rather than at a distinct point where her head should be - an as though she’s standing upright.
It was actually funny that so many people were joking about the grim reaper when it first showed up, since Oda was dropping hints from the start that something was up. It’s rare to see Zoro terrified, much less plead for mercy, and Zoro saying “who the hell are you” when it shows up suggested that he’s actually worse off than he was at the end of Thriller Bark.
I also think it’s an interesting development since the “impossible” aspect of Zoro’s dream that many Straw Hat dreams share was never whether or not he can beat Mihawk, but whether or not his accomplishments can transcend the mortal coil and reach the afterlife. It’s never really been addressed directly, so I wouldn’t be surprised if that‘s what it’s leading to. There’s also those dangling plot threads regarding Zoro visiting Ryuma’s grave in Ringo, Ringo being one giant graveyard, possibly more Onimaru, and the running theme of people going to Ringo when they’re near death. So it seems fitting for Zoro also being at death’s door.