No, it doesn't get better.
Season One is GOLD! I love every episode and love the pace/timing of Tomlin and Fonda as they embark on their new lives. Season finale was truly worth waiting for season two for.
Season Two was just okay. Less good pacing but still a solid story, especially with the addition of more Ernie Hudson. The season finale was a bit hokey but I took it as the writers were letting it flush itself out.
Season Three was a LETDOWN. Frankie's meltdown season. Too much emphasis on the ex-husbands. Not enough emphasis on their friendship, and a crash and burn of their professional relationship.
Season Four JUMPED THE SHARK. Turned into a 70s style soap opera with the addition of KIDS to bring in the awe factor. Lost its original point and meaning from season one, and is the one season where you don't give a rip if they come back for season five. Felt like they were just selling Fonda/Tomlin and not the show.