I don't really know the story behind all the different actors for her parents (as in, why they kept having to re-cast them), but yeah, I do find it annoying when that happens. Would've been nice if they had managed to cast people from the start who could've stuck with them throughout!
It's not really annoying, it's just based on how long the actors are willing to stay around. Topanga's parents aren't in the script as much as Cory and Shawn's parents. Just as long as あなた have someone to fill in the script and play the characters. I thought I saw Annette O'Toole play her mom once, she's a wonderful actress.