Season 5 had some excellent episodes: "The Great Petrie Fortune"; "See Rob Write; Write, Rob, Write"; "The Man From My Uncle"; and the unusual "Buddy Sorrell, Man and Boy".
But there were some unsatisfying episodes as well: "Draw Me a Pear"; "Viva Petrie"; "You Ought to Be in Pictures".
However, all the seasons had their good and not-so-good episodes. What made Season 5 a little different from the others is that the series began to have a slightly different feel.
Seasons 1-4 seemed to be more or less frozen in time. With a few exceptions, you can watch the episodes from those seasons in random order. But in Season 5, things started to change some. It's as if everyone became a bit restless. Rob's hair was longer; Laura adopted a very different hairstyle in the middle of the the season. Rob's focus had always been on his home and his job; now he ran for city council. An old friend of Laura's moved, with her husband, into the neighborhood -- seemingly threatening the bond between the Helpers and Petries. Alan Brady acknowledged that his show would someday come to an end. Richie was well past the age when he was the cute little kid singing "You're the Top".
Did the Dick Van Dyke Show ever "jump the shark". No, it never did; but I have a feeling it might have, if it had continued on to Season 6.