Misty has appeared in about 42% of the episodes. How could you say she is not a major part of Pokemon?
As a character she was about as important as any other character.
In terms of the overall series, she's in less than half of it. The show is currently in its 14th season, Misty was only in the show for 5 short years.
She was the shortest lived Kanto character of the show. Ash, Brock and TR are all in double the amount of episodes she was in.