Are you using PC, Chromecast? Is one better than the other? Wired or wireless? I get excellent stream quality with Chromecast wired, but some of my PCs for reasons I haven't pinned down have trouble with a good stream on Chrome even though they are also wired.
Stadia presently only supports a 1080p stream on anything that is not a Chromecast Ultra. Therefore, on a PC in a Chrome window, for example, Stadia games will look noticeably worse than they otherwise would. I can easily tell the difference between my 4K TV streaming at 4K and my 1440p monitor that uses a 1080p game stream.
Destiny 2 on my PC looks a bit blurry, with obvious aliasing. On my TV, it looks pretty good via upscaling, though not as good as a high-end PC running it at 1440p or 4K natively.
Providing informations about your setup would help a lot here. Are you playing on tv, chrome browser? Connected to wifi or ethernet? Ect.