Haven’t you asked this before? Have you tried any of the special scaling effects/filters I suggested?
In my answer to your first question I said that scaling with the Transform and Position and Zoom effects cannot be (further) controlled wrt the algorithm(s) used. Kdenlive uses the MLT Framework for effects, so you need to ask the MLT devs whether they will change the filters to allow for that. Until that happens you have to either suck it up or try using other scaling effects like I suggested.
Also keep in mind that unless you render with a lossless codec, the video encoding will always result in some quality degradation simply due to the way (lossy) encoding works. You can try to play with the quality settings and different encoders (some are better for movies, other for animation) to see whether the results are more what you want. Higher quality encoding mostly creates bigger files, though.