While working in the Photo Editor, your text layers will look sharp and crisp as individual layers. However, once you flatten layers on a project, all layers will be rasterized and merged to your background.
When a layer is rasterized, it's converted from a vector (i.e. elements like text, shapes, or graphics that can be scaled to any size without losing quality) into pixels which repopulate with the rest of the image. Since you'll start to see the pixels of an image when zoomed in past 100%, the same will be true for those text layers you flattened to the image.
Before and After Zooming in on a Flattened Text Layer
Your exported project will become fully rasterized upon saving as JPG, PNG or PDF. It should be known that when you preview your exported image, it's normal to start seeing the individual pixels of an image when zooming in past 100%.
Pro Tip: It's recommended to export text-heavy projects as PNG or PDF files for best results.