This is definitely an ongoing issue. Both when working in Spark and also when downloading the image. I have this issue on my MacBook running Catalina in 3 different versions of Firefox. Also have it on my Linux Mint machine in Brave browser. Thus far the only machine that it isn't an issue on is my older MackBook running Mavericks and Firefox.
Definitely seems like there's some kind of issue with newer versions of Firefox, too. I agree its frustrating. I feel like I'd be better off ditching Spark in favor of Photoshop. I can understand how if I'm going directly from my Stock Images to Spark there could be issues, but I've also had some really bad resolution issues uploading a Stock Image to Spark. I'd rather not have to use multiple computers for a project.
Catalina & Firefox- low resolution when working and when downloading
Linux Mint & Brave- even lower resolution when working (even from uploaded image) & when downloading
Mavericks & Firefox- good resolution when working & when downloading
Text is typically blurry & pixelated in raster images, especially low-res ones. And the undesired effects are magnified on hi-pixel density devices like Retina displays. That's the nature of the beast. Unfortunately there's not much you can do about it on a closed platform like Spark.
All I can suggest is that you use different tools to make your social media graphics. If your social media channel supports SVG, use Illustrator to create vector graphics and export to math-based SVG which retains sharp, crisp lines on any device (see screenshot).