As if having one Ataru wasn't bad enough, now there are two of them. And they were polar opposites to one another.
Ataru's mother might say that she wished she never had Ataru... but she's still his mother. Not being selfish and actually wanting her son to go back to normal is motherly. Even though she might regret this decision later.
I feel like some people overemphasize the lack of explanation for what happened after the cliffhanger endings. Yeah, it'd be cool if they explained how they got rid of the problem sometimes.
But for situations like this, you can easily say that the effect wore off after a while and people went back to doing their things like nothing happened. I think most of these stories would end anti-climactically like that if they were extended.
And craziness like this is a normal and regular thing in the Urusei Yatsura world. So it wouldn't make much sense for them to reference past events for no reason.