Yep
She's just attached to him because of their history together, and I'm positive there's nothing romantically-intended in her actions. At minimum, nothing she's actually cognizant of.
He was high on his own supply being good at things and always being the one to help her out, until she suddenly got better than him, and instead of using that as motivation to push himself, he just threw it all away in frustration and ran away from everything.
And Sayaka's clueless about it, most likely, because he also stopped talking to her in any meaningful way, and she arguably doesn't realize anything is "wrong" at all, and assumes it's all the same from when they were kids. He's done nothing to enlighten her, and instead is freaking out on Puberty Brain as he listlessly exists through the day with nothing really motivating him to change his situation.
I thought I'd really dislike his mom's attitude toward him, but honestly if he's been like this for a year, then she's probably just lost her patience, which I can understand.
She's definitely not helping matters by acting this way toward him, and thus I'm of the opinion that they both kinda suck.
But the onus is on Kazuki to grow as a person, and if his mom comes around after he starts making an actual effort, then fine.
Sayaka's personality being this kinda-ditzy-but-somehow-perfect is real high up on the Wish Fulfillment scale of "amazingly hot high-spec childhood friend but she's still clingy with me specifically even though I push her away constantly", but what can you do.