Yes, this may be an app issue, not a hardware issue. Especially if you find only certain apps currently have this problem. Eventually may be resolved once apps are updated:Apps not optimized for the new mini 6 screen resolution are downscaled by ~3 % to fit the narrower width (reduced by 48 pixels compared to the previous models), which may explain that it appears less sharp.
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Get ready for the new iPad mini display - Latest News - Apple Developer
The new iPad mini introduces a new screen resolution and aspect ratio with an edge-to-edge 8.3-inch Liquid Retina display, enabling even more immersive app and game experiences. If you’re following best practices with Auto Layout and size classes, your app’s interface will dynamically adapt to...
In any case, the Mini 6 display is 3% narrower in portrait so a page's text will be 3% smaller in portrait. Just the way it is. But it gains 10% in landscape. So pick your preference. I wish the portrait width had stayed the same, and only the landscape increased, which would have been a win-win