The first Smash Bros. came out for the Nintendo 64 in 1999. The latest Smash Bros. came out for the Wii in 2008. So of course, the differences in looks are rather dramatic.
This video from YouTuber Master0fHyrule focuses on how the original twelve Smash Bros. characters' taunts have evolved over the years. However, it's also a very effective comparison between the three games' graphics—especially the character models. It's quite the trip across gaming history: