Pages does not provide user DPI adjustments. The best that you can hope for is to create one-page artwork in Pages, and then File menu : Print : PDF : Save as PDF. You can then drag/drop 1-page PDF artwork onto Pages as your first page, and on export to ePub, check the box indicating that you want it to use the first page as your cover.
If you don't want a final PDF, then File menu : Print : PDF : Open in Preview, and then choose one of Previews Export options. Drop that result image back into Pages.
To achieve the sharpest results, you need to avoid Text boxes (transparent backgrounds), table cells (transparent backgrounds), borders of any kind, drop shadows, reflections, and any deliberate changes to opacity — all of which will render as raster 72 DPI within that PDF.
For me, if the image has be absolutely its sharpest, I do it as all vector artwork and text outlines in Affinity Designer, and export as a 300 DPI PDF, or Photoshop (.psd) file.