>>6962410>>6962388>>6962379>>6962348It's a fake mouth.
There is no real hole there.
Pretty much every artist who is familiar with the character knows that there's a human in there and drew him as such.
See also Carnage on how he has that pink mouth, which is usually shown as actually being flat, since side views also show pink instead of being able to look through the other side.
Artists who aren't familiar with the character, like Yamaguchi, just do whatever they feel like, even when they say its based on a specific design like Ultimate Venom.