Something happened with Teri/Lois beginning in early season 3 that went beyond just the haircut. The haircut might be the most obvious sign of the change, but her wardrobe went to the proverbial zoo around the same time. The metallic silvery getup in WHALTTA... the red & white-stitched thing that graced us in two (!) episodes -- I recall giant buttons on another one -- and a lime green ensemble somewhere in there. She didn't seem to settle down until mid-season. (Rumors at the time seemed to implicate Teri in this, saying she had demanded Lois wear more "stylish" clothing. But there were a lot of Teri rumors out there, many of them not entirely true, so this is all hearsay.)
Even though Lois's outfits went back to being serious in season 4, her makeup took a strange turn. There are some eps where her eye makeup is so pale she looks ghostly, and would probably have looked better with none at all. That, coupled with the short-short hair (and the unfortunate wig in the early part of the 4th season), definitely made her look different. But like others have pointed out, she ACTED different also. I remember at the time, FOLCs were complaining that Lois rarely did anything "Lois-y" anymore. The whole under-the-table/chopsticks scene in AKAS was lauded at the time for being the first time in AGES we'd seen her being HERSELF. And that may have been the last time, too.
So yeah, it's pretty easy to associate her different looks with different phases of the show. Kind of as markers, as someone said. But in the end, even if her hair had stayed the same throughout the four seasons, we would've still seen her personality change -- unless we'd had different writers/producers/execs.
So no, it certainly wasn't the hair's fault.
