One show that really surprised me with how much it improved after a mediocre start is "Parks and Recreation." The first season was pretty rough and felt a bit aimless compared to "The Office," but once they hit Season 2, the characters became much more fleshed out and the humor sharpened significantly. Leslie Knope evolves from being a somewhat quirky yet annoying character to a deeply relatable and inspiring figure.
Another one is "The Good Place." The first few episodes set up the premise well, but it really finds its stride midway through Season 1 when the twists start to kick in. The way it plays with philosophical concepts while delivering solid comedy is both clever and engaging, plus the character development is top-notch.
Lastly, "Brooklyn Nine-Nine" is a great example. The pilot was fun, but it took a little time for the characters to find their rhythm. By the end of Season 1, it has some of the best comedic moments and character arcs in the series.
If you're willing to give some shows a bit more time, these are definitely worth it!