Anyway, here's a visual comparison compared to the first game:
https://www.youtube.com/watch?v=W0TXd5w_EuITL;DR splatoon 2 is much more full of life and detailed than the first game. in comparison, the first game looks dull, with very simplistic animations and jagged edges from the 720p rendering. on the other hand, the second game looks very colorful, with very vibrant animations, more details, and smoothed out graphics due to the 1080p rendering.
Edit: I'm late, but I'll leave this here for the summary.