You said "low quality". Forgetting Plex as there's a million reasons why quality degrades when you're transposing and streaming remotely.
Using Kodi you're on your own LAN, so no transposing is involved and it just plays the file as-is.
So, you're down to quality of the source, settings on the video card, settings on the TV, etc. LAN problems with Kodi would usually be Buffering, or blockiness etc.