GameXplain have made a wonderful video comparing the graphical differences between Super Smash Bros. Ultimate and the series’ previous Wii U iteration.
A big difference you will notice is that Ultimate has a more coherent style across all the characters, making them all look like they belong in the same game rather than a mashup of different art styles. There are also lighting changes, along with some minor tweaks to the stages.
In the video below, you will see a few of the characters side-by-side, including Link. GameXplain went to the trouble of getting the characters on the same stages in the same or similar poses. There isn’t an in-depth analysis, but it is still interesting to see the differences between the two versions.