I dont think the author is excusing the mom at all, rather the message is despite whats happened in the past you need to try to move on or else things will just get worse. I doubt the intention is to make the mother come across as a victim.
The problem is that it does. Because for the mom to be able to come back she has to acknowledge that she did something wrong. And unfortunately trying to justify or excuse her behavior comes across as mitigating her personal responsibility for what happened and putting the burden on Kohaku to bear all the weight of solving this through simply choosing to forgive and forget.
It also, sadly, kind of feels like an extension of that whole extremely Japanese concept of Filial Piety or whatever, where kids are expected to have infinite understanding and acceptance of however their parents treat them because "they must've wanted to do good for me and I need to be grateful or I'm a horrible person" crossed with the excessive push for non-confrontation through brushing things off or letting them go. It comes across like suggesting that no matter what her mother did, if Kohaku doesn't just smile and say "it's OK if we can be a family again" then she did the wrong thing. And it is a massive disservice that she is made to feel guilty as if that's a worse thing than her mom actually admitting her guilt for, and this cannot be stressed enough, striking her toddler daughter hard enough to require an ambulance and then using that as an excuse to continue to psychologically/emotionally abuse her for like 14 more years or however old the characters are supposed to be.
Moving on/getting past things requires addressing and working through them to a satisfactory degree. Forgiveness first needs contrition. If the mom had broken down and taken total ownership of being awful and then decided to stay away from Kohaku for that reason ("I hurt you and if I stay I'll just keep hurting you") that might've helped. But the way she leaves it's more like "you being here forces me to remember all the awful stuff I did, so I'll just get away from you so that I can choose not to deal with it." and that is not laudable at all.
This series was fine but a little iffy for the first 7 or 8 chapters. Then when it seemingly became clear that it was just going to be a LN/WN advert it went downhill by focusing entirely on the wrong thing in the worst possible way. They would've been better off ending on the two of them still hanging out at the restaurants and vowing to work through their family issues together without confronting them yet. Better to let teh audience imagine the author could write their way out of this hole than explicitly show us that they can't.