The show was full of nostalgia in the early seasons. The producers worked hard to make the show look as authentic to the 1950s as possible (though there were occasional glitches, like the previously-mentioned flag having the wrong number of stars).
But as the seasons passed, the producers and other staff got lazy, and stopped paying attention to wardrobe and hairstyles. Erin Moran's goofy 80s perm, Chachi's... well, everything about Chachi, and the new characters who were added later had nothing to do with the 1950s or the early 1960s.
I only watch the older episodes.