Why does playing back video in Shotcut with GPU acceleration make things like text look jagged?
Did you try improving the deinterlacer and interpolation options in the player settings from the menu?
The jaggedness is usually a result of the player compromising a bit of the playback quality for speed. Depending on what’s in the video itself can make this look super obvious, like in text or extremely fast moving objects. Improving those options will slow down playback (unless you have the realtime option on, which is default, but you’ll get more dropped frames) but you’ll get a more accurate representation of the video quality. However these have no impact on the final exported result, so it’s all just a matter of preference and what your computer can handle.
Edit: I’ve changed your post title and category to help/how to
I’m using Lanczos and temporal + spatial. It won’t affect me too much if I have to rely on the CPU for playback. I just want to utilize my GPU in some way.
I don’t know why using GPU acceleration specifically causes it. If you’re already using the highest settings then I don’t know how else to help you. 
Do you think it may just be that video file itself then? Or does it look fine after exporting or in some other program?