Like most anime, there is no high def master of this. It was animated in standard definition. Where blu-ray helps with these sd upscales is minimizing loss of detail from the master and eliminating compression artifacts. It can't however eliminate line alaising (jaggy lines) as they are inherent to the sd nature of the master. This is an earlier funimation release when they weren't putting lossless native language audio on their releases. Thankfully they have changed this practice with more recent releases.
Short of re-animating in HD, we are not likely to see better releases of most of these series. The only improvement we could hope for would be the inclusion of a lossless native language track.

DVD Upscale video quality, no lossless audio for Japanese

PQ is DVD upscale quality, not true highdef. The style of animation may make this a moot point as there is very little in terms of shading, detail, or depth in the drawing.