FCP retime timelapse image gets blurry
If you retime the images so that there are more than one images that can land on a frame, your video software will likely interpolate between the two by blending the two frames.  If you play back at 12 fps with zero offset, your video software should play the first image for two frames (assuming 24fps playback), then the second frame for two frames, etc.  If you play back starting with a one frame offset, the first image will play for one frame, then a blend of the 1st and 2nd image will play the second frame, then a blend of the 2nd and 3rd image will play the third frame, then a blend of the 3rd and 4th image will play the fourth frame, etc.  That image blending will make the film look blurry.

Alternatively, say you have 240 frames and you want to play them at 24fps, but you are asking the editor to play them all in 1 second instead of 10 seconds, the video editing software might not pick every 10th image and display it for one frame, but will instead blend 10 images and play that as a frame.  That could also account for the blur.