So the 1080P videos don't looks sharp and Full HD in the 11" iPad Pro in YouTube ??
Correct - sorry if I did not explain well.
The iPad Pro 11" screen has a much higher resolution than 1080P (1920X1080), so when you play content on it lower than the native resolution (2388X1668, or almost 1440P), it looks bad. This phenomenon is not unique to iPads, but that is why it looks so bad in this particular case. If you're at a computer right now you can see for yourself - set it to a much lower resolution than your monitor's native resolution and watch how much sharpness you lose.