My suspicion is that she's kind of emotionally damaged and in need of affection similarly as him. Could be from something as simple as that she's been doing a demanding job for 20 years without being able to have a proper relationship and miss intimacy and closeness with someone, to having lost a baby to a missed abortion or something 15 years ago that traumatized her.
Especially since she switched gears completely from wanting to undress him and make him take a shower and started doing mommy stuff instead when she found out she was underage. I think that there's some other psychological mechanisms at work here than solely being a sexual predator.
I'm hoping it's something similar to this, more a bait and switch by the author. It would be interesting if he plays up this suspected predator factor only to reveal later that she really does just want to be a good mother to him, but just the timing and circumstance leads to unintended drama. I think it would be very interesting if it's revealed that she legit had a come to Jesus moment after her night with him and decided to herself that she wanted to quit prostitution (I think that's implied? Or maybe the milf hunter app is just for quick sex?) and be the kid's mother. However, we're suspicious of her because of the mysterious events of chapter 1 and author frames things a certain way to mess with our minds and create tension.
Not only that, but we see it through the lense of the hormonal boy, who has never had a mother and whose father neglects him. He's probably an outcast by his teachers at school too after the sugar momma thing, so he's had no adult role models in his life, thus giving him no frame of reference on what it's like. Any level of affection he'd receive from a beautiful woman is gonna mess with his teenage mind because he doesn't know any better. Plus he literally met her through "milf Tinder" so rather than her actions coming off as motherly, he'll also only see the prostitute/easy woman in her, which is why the story frames their interactions as more creepy and sexual. But maybe it's really just that he's got no experience having parents and she has none being a proper mother. Not only that, but being a prostitute for so long could mean maybe she struggles to turn off her sex appeal, so all her motherly actions just come off as creepy. Then we find out later that it's just how she acts along with how the author framed it from the boy's perspective to bait us.
At least I think this would make for it to be a way more interesting thriller/drama than just "mommy pdf file" trope. There's plenty of hentai like that out there already I'm sure.
Playing up his perspective that she's a creeper for a while, only to show later that she's a good person trying to get out if a bad occupation and to be a mom would be a lot more interesting of a case study on childhood trauma and parenting imho. There's more directions they could go with the story after that too since they'd still have to keep her identity as his onetime sugar momma hidden from the school/his father, and maybe they could have people from the mother's dark past show up and cause trouble.
I hope the story goes more this way instead, but I could have just written this entire speculative novel only to be completely wrong and she will just be a yandere pedo. Guess we'll see with more chapters haha.