I like Jo at certain points in the series, and find her really irritating at others, and I suspect this may be the case for quite a few people. I like Jo as a Middle and in the war books as a young mother, but as a Senior I find her very negative - snappish, irritable, obsessive about the Robin, etc. - and post Canada she's just too over the top in general for my taste.
i'm the same, I love her as a young mum and a Middle, but not so much when she's a Senior or in the Swiss books, and I have the same problem with MLT when she's a Senior. In Three Go, however, she is absolutely delightful and makes me laugh (and EBD was, I think, really good at writing tween girls -Lavender Laughs is another example).
Also, in Joey's defence, it's Jack who babies her and insists on Not Disturbing Your Mother a lot of the time, it's not really her fault. Joey is very tough as a child, mentally if not always physically. The girl risks her life G-d knows how many times for other people, from Grizel to Maureen. I think he underestimates her. I've not read Theodora but apparently he and Miss Annersley refused to tell her about Margot - why?! She's got every right to know about her children and Miss Annersley knows Joey well enough to know that saying things like 'it was bad' will just make her worry even more.
And EBD's created a world with loads of characters in, many of whom are likeable, some of whom aren't - Daisy, Tom, Cornelia and Bride are among my favourites. Sometimes supporting characters end up becoming even more popular than the mains.
Joey obviously wanted her girls to share the positive experiences she had as a school girl. I think it gives a nice balance between the freedoms of school and the inevitable child care responsibilities in the holidays, rather than them being on older sister duty 24/7