She may not be everyone's cup of tea, and she's not my favorite character by any means, but I appreciate what she does and how she is for the MC.
Definitely don't want to minimize your own life experiences, but it's worth pointing out that you can both be sort of right about this. Because it feels like you're coming at it from different angles.
It's entirely possible to say that Opal is a caring, loving, supportive mother who wants nothing but the best for her child, and who certainly tries to be the best mother she possibly can. It's entirely possible to point out that she is, in many ways, a
much better parent than parents in couples who are harsh, abusive, or otherwise terrible. A financially stable same-sex couple can just as easily be horrific parents as any other.
But it's also entirely possible to say that she is still somewhat irresponsible for deliberately attempting to be a single mother without any real support structure to help (as opposed to being thrust into single motherhood by death, divorce, etc). She chose to fulfill her own desires while ignoring or downplaying the potential negative impact that would have on her child. Especially when she
would absolutely have to work hard to support two people without financial support from a spouse, parents, etc. One could argue that your character was
EXTREMELY lucky to have moved into a cul-de-sac with very supportive families and kids your own age - if circumstances were different (say, her job sent her to a different town), you could just as easily have wound up living in a neighborhood with no kids, where you have to spend most of your time cooped up in the house as a latchkey kid waiting for mom to get home. The MC could have been incredibly lonely, depressed, and utterly miserable, all because Opal wanted to be a mom.
The problem with "wanting a child despite not wanting a partner" is that it is, at heart, an inherently selfish act. At least in the sense that you are to some degree putting your own wants and needs ahead of the potential impact it will have on the child in the future. Without even getting into sociological or psychological discussions of intra-family dynamics, the financial situation alone is an issue. Sure, she starts off with what seems to be a stable job, but what if she was fired and couldn't find a new one? What if she got too sick to work? What if
you got sick or injured, and she couldn't afford treatment or medicine? Buying clothes at the thrift shop isn't an inherently terrible thing, but it dramatically increases the odds that you're going to get picked on at school for being "the poor kid". And as we've seen, her having to work late (thus potentially leaving you alone or missing incredibly important milestones) is a regular enough occurrence that it's an issue - one that might have been greatly offset if you had a father (or another mother, or a live-in aunt, or whatever).
As your own character can point out in the homeroom scene when the teacher asks everyone to hold up their phone, that would be an
incredibly embarrassing moment for any student who didn't have a phone (congrats on spending the rest of the year getting picked on for being the lame poor kid!). And you almost
were that kid - as tight finances and fear that you weren't responsible enough to get one earlier (ie, you'd probably break it or lose it and all that money is wasted) means you didn't get a phone until shortly before school started. That's another tangible example the game gives us that, while you aren't starving or homeless, money is definitely tight enough to cause noticeable issues.
And the fact that you had to move to the cul-de-sac in first place is tied to her fiancial situation. Which resulted in her having to completely uproot you from the life you'd already made elsewhere, whatever friends you had, everything you'd ever known. That can be incredibly traumatic for a child - and while it's certainly something that any couple (or widow/widower/divorcee/etc) might have to go through as well, deliberately choosing to be a single mother dramatically increases the odds of it happening (and the MC is lucky it only had to happen
once - some families wind up having to move every few years).
The fact that a financially well-off hetero couple can beat, bully, abuse, or utterly scar their children by being terrible parents doesn't necessarily mean Opal
isn't an irresponsible parent because of the situation she herself deliberately created. Both can be bad in different ways.
That being said, I'd argue that a lot of whether or not Opal is a bad parent is going to depend on how you play your own MC - an MC who picks all the "obviously mom loves me and I love mom" dialogue answers is clearly in a much healthier place than one who picks a lot of the "I hate mom for caring about work more than me" options.
And there's no real wrong way to interpret it. Maybe one kid is just like "this is the way things are because it's the way things have always been", and are very resigned to the idea that they don't get fancy clothes and lots of vacations (and maybe they resent Baxter for being rich because of it), and are very supportive of their mom because they feel like it's "us against the world" and "mom doesn't want to work all the time but she has to because she loves me and needs to take care of me". While maybe another version of the kid is very much aware that "we're poor because mom was selfish, and now all the other kids with fancy toys and new clothes and big families get to have
so much more than me it's not fair", along with "I wish I could have a dad like other kids, but mom ruined that too".
Personally, I played it more like the self-sacrificing little martyr most of the time who makes excuses for it (and who comforted mom the few times she gets upset about things), which ties in to being the most conflict-avoidant kid in the entire school (always trying play mediator and conciliator). But now the diner thing is kind of the last straw (especially with Tam and Qui constantly pulling apart while I keep trying to drag them back together), and I'm going to be a sulky little tantrum-thrower for a bit. But I totally understand if someone else is going to play it as "Opal is a monster and I'm going to treat her as such" (especially since we
are talking about hormonal teenagers here).