NSMBe's import bitmap feature seems to have a lot of trouble dealing with gradients.
Here's the image before importing:

Here's the image after importing:

Here's what the palette looks like:

As you can see, there was a pretty big loss in quality after importing despite there being plenty of free space in the palette. I'm aware the chances of this being fixed are pretty slim, but I decided I might as well report it anyway.
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Unfortunately, that's not a bug.
NSMB uses only 5 bits per RGB channel (32 values ​for each color) while any other image uses 8 bits per channel (256 values ​​for each color)

ah, I see, so I can't even fix it using another editor
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.