Could be a gamma issue. I had this problem on GLES2 Android, where the brightness (really the gamma conversion) was totally different.
In general, though, every single monitor (even two monitors that are the exact same model) do not have the same gamma. They are factory adjusted, but even so, you could buy two of the same monitor and put them next to each other and they will look different. I doubt that is the case here, since Apple makes the displays on both the MacBook and iPad and they are usually good regarding calibration. But it is something to keep in mind, especially when you support cross-platform and have no idea what displays your users will have.
I don't think Godot allows you to control the gamma/sRGB settings in the editor (project settings). However, if you are using shaders, you can override the functionality. If you are not using shaders, then you can adjust whether sRGB is enabled or disabled on the image. This is in the Import settings. It defaults to Detect, but it can make mistakes, so you might want to force either Enable or Disable, depending on what you are doing. In general, you do want sRGB, but it's more important for 3D games. With 2D, you can always disable it and adjust the colors in GIMP/Photoshop if you are having issues.
