With TV shows, this isn't that surprising. For your typical broadcast shows there's usually one of two scenarios....
1) The first season is of a set quality. That quality is either too poor to generate a second season, or it does, and the second season is poorer and the show usually dies during/at the end of that second season.
2) The first season is of a set quality. That show is of sufficient quality to generate a second season which is of better quality, and the show continues. Repeat until it declines.
There are exceptions, but I have a hard time picturing any show that went 5 or more seasons where the general consensus is that the first season was the best. Likewise I wouldn;t expect the last season of any show going that long to be the best. Enterprise seems to be an exception in some people's opinion.
I think most people feel Thor: Ragnarok was better than the first two. One of the few example I feel that way about in movies, esp. trilogies.