Sounds like a normal thing that happens. Premier does that to allow for playback to be smooth on systems; There is three things to check if your system can handle full quality playback, you can enable.
First, on the program monitor ensure:
In the view menu, when your active window is the program monitor, change the playback and pause to full or what ever looks good anbd works for you:
And, again if your system can support it, turn on the "high quality playback"
As Peru Bob also suggested, rendering the timeline can also improve the playback so its not blurry. The bottom line is you have to play with all these levers based on your system if you want things to be 'perfect' while previewing your work. Your always making a trade, quality for speed. Premier lets you tune those choices.
If, at the end of the day, I suspect your concern is on export that the text will be blurry. If that's your true question, the short answer is it will not be. You can export your work, and play it back it will be fine. When you preview it without pre-rendering, premiere builds the scrolling text realtime, and it seems to have decided to lower the quality to keep the video smoothness on your system. You can tweak the above settings and if your system can support it, it will play back perfectly clear if you don't have to much happening on the timeline while playing that part of your project.