It would have declined and fallen in its 3rd season regardless of who played Batgirl, or whether the show had introduced Batgirl at all.
We don't not know that--all we know is that Craig was the one cast as Batgirl, and her less-than-heroic performance, and not a shred of believability as one who could stand up to villains killed the series. Whatever was poor or retread coming out of season two was no longer a factor, as the dynamic of the series drastically changed with Craig's laughing, stubby joke of a superhero.
Come on, she was slowly delivering high kicks a guy using crutches could avoid, and without being rich (never implied on the show) exactly how did Barbara Gordon build all of her gadgets and that motorcycle, which was ouftitted with more than a few then-high tech devices?
At least with Bruce Wayne & Dick Grayson, the argument could be made that with their millions, they could pay for the elements which made their car, computers, etc., but Barbara Gordon did not have that resource (she was a librarian).