Our resolution on the streaming app recently got much better. Looks almost BluRay quality. We only watch the Food Network this time of year, so I don't know if it's just that channel, but it sure improved. I still wish they'd stop down-mixing the audio to stereo, because I can understand dialogue better when it's 5.1, but at least the image is better now.
Also, I recommend doing a hard re-start on your Roku. Don't just unplug it, find the restart option in the settings menu. They did a firmware upgrade a few days ago, and it didn't take effect until the restart. That's unusual, but there it is.