The skull block is commonly found in SMW rom hacks. It's most commonly used as a death block (act as 12F) that looks clean (no floating munchers or ugly blocks).
Also, this is my first ever submission!
While this type of block could see a lot of use, just having this simple design alone is not enough to warrant a full submission. In fact, it's because it's so simple that it's easy to make by oneself, and that's why it's so common elsewhere.
That said, something like this would be very welcome if you could add more to it. Why not make it something more like a terrain that has edges and boundaries? Maybe some simple shading as well.
No Sample Level
Normally simple changes wouldn't need a level, but it is ExGFX (as opposed to a GFX change), and someone probably won't know what the proper graphical slot to replace is.
If you decide to make it less simple like I suggested, it most certainly would need this, so keep that in mind.