Studio Visualizer is showing the images/ graphics as low res while they are in fact high res.
A blurry or up-sampled representation of your design or of the images in your design is displayed in Studio Visualizer.
When you open/import a design, it will be rasterized to bitmaps of maximum 2048px on 2048px (Image Quality: High). So, for larger jobs the result will be more noticeable than for the smaller jobs.
Check whether the trim box is only covering the design used in the Studio Visualizer, so without any unnecessary white space or artwork info panels. If necessary adjust the trim box to be as small as possible so that the design will have a better resolution when rasterized to bitmaps of 2048px on 2048px. If your design is i.e. 100mm wide and 20mm high, this would by default result in bitmaps of 2048px on 512px.
You can change the resolution of the bitmaps in Studio Visualizer by using the Very High Quality entry in Model > Image Quality. This will completely re-rasterize your file to bitmaps of maximum 4096px on 4096px. This requires a computer with enough RAM and it is best to apply this as a last step just before you export.
If your normalized PDF file contains external references to images, then Visualizer will only use the 72 DPI image preview embedded in the PDF. To resolve this, just write a normalized PDF with embedded images and use this normalized PDF file in the Studio Visualizer.