I think the writers still love Mona and Paige and Melissa and all the others, its just they've got so many characters and storylines in play and only a limited budget and time that they just do a couple of eps story, leave it for a few eps and then pick it back up again. I mean Mona had some awesome stuff in the premiere and the eps which left her back in Radley again. Now she's back there, she's basically "on ice" until they need her again. I think because she's so competent and intelligent they find it hard for her to be around all the time because she'd solve the ep whilst the liars were still at The Brew.
I' more annoyed about Caleb, who disappeared for several eps, and has been barely involved with Hanna's storyline except to be the voice of reason once and ep. I guess they think if he's leaving to be the lead character on Ravenswood and there's big blow up between him and Hanna then they've got 10 eps of a new show to feature him, but its sad we aren't going to get much more Haleb time ever.
It is really, really annoying that they still find time to have Ezra moping around, but its really not doing the character any favours and Love interests have always been 90% of Aria's storyline anyway. Sigh.