If you are using a hi-rez texture pack, but your favourite server uses some textures not supported by the pack, then you may see lo-rez textures alongside your nice hi-rez ones. In MT 0.4.13 this can be fixed by setting the "texture_min_size" parameter in your minetest.conf file.
If you set texture_min_size=16 (default value is 64) then all textures 16px and above will have filters applied. The bi- or trilinear filter _must_ be enabled in your settings for this to work. The result of this is that the lo-rez textures are upscaled and blurred, and will now look much better next to the hi-rez ones.
Note: earlier versions of MT, (0.4.12 in my experience anyway) did this automatically.