Kind of hard to narrow it down to a specific year/season, but i would not call her major in the 70s. Yeah, she had the pageant sketches but really that was the main thing she had for her. Aside from the pageants and the difference in hair styles there was very little distinctions between her and Betty Lou or any other random "little girls".
Sesame did have the (valid) criticism for years that there were a lack of female Muppets and i think they got more proactive towards addressing that in the 80s and you saw Prairie more - but i'd still say she was reflected more in books and merchandse than the show itself (course that was also especially true in the 70s)
I'd still wouldn't call her a major character in the 80s. The most prominent female Muppet, yes - major character, no. But she was used more and more of her personality began to develop. The 90s is when she really took off. Yeah, there was also a push to include more females like Zoe and Rosita (and briefly to resurrect Betty Lou and give her some more distinct personality traits) but even with the big media hooplah with Zoe, Prairie really started to come into her own - featured more in street stories, pairings with Monty, Cookie Monster...
It's been a slow climb to the top (and when one considers the "top" as Elmo, then Prairie's still climbing - Abby Cadabby got there much quicker and sooner - but hey, she was wings which makes it easier to reach high places, right?), but Prairie's rise has been slow and gradual. Mileage may vary on when she finally crossed over from minor to major.