I haven't played it myself so idk if I'm being entirely accurate, but I think
that the background you've got here is actually the one from Super Mario All Stars?
As a reference, the original
SMB1 1-1 background can be seen here:
Speaking of which, I don't believe that many backgrounds from the actual SMB1 have been imported. This is mainly because the graphic style of All Stars fits more with the overall style of SMBX compared to the original SMB1.
Anyway, I like the look of this, and it'd prove useful when creating an SMB1 overworld without the need of clouds floating around.