I recently designed a simple profile interface just for practice, uploaded it to Roblox as a decal and its quality is awful. In game its quality looks noticeably worse than the attached but below is what it looks like in game. - Do ignore the text unalignment.
In my opinion, I think your better off remaking this with Roblox’s UI elements. UICorners should be able to help out well with this type of design. If you use it right, you can also make it scale better than the decal, since it can get stretched and such. (however, you could just use UIAspectRatioConstraint to not make it stretch). The dotted pattern could just be exported alone. I know this is not related to the issue, but I’d still like to express my opinion a little.
As to the problem you’re having, I think it’s related to this;
I did actually also try exporting at 1024x1024 but the quality was still subsidiary.
I also to add used a pixel bleeding tool to help the quality - removing the white border but as for quality it didnt help.
Roblox does not support rendering graphics with a resolution higher than 1024x1024 YET.
If you want your GUI to look better, I suggest you make the GUI in Roblox instead.
Making frames with rounded edges shouldn’t be too difficult and the little icons and badges don’t need to be that high-res.
You could make a sprite sheet of icons to select from or upload everything as multiple, smaller images that can be composed together in-studio.
Using Roblox built-in text labels and whatnot should also result in much sharper and more readable text since it uses vector graphics internally.