I don't see how it would be lazy writing if it becomes necessary and someone is capable of doing so. In that case you could argue that Roxy appearifying a new Matriorb is lazy writing too. Or Jade saving the planets from the Beta session. Or John retconning the timeline.

I’m no English major so this isn’t a perfect analysis but,

It would be lazy because there’s been no build up to it. Creating a new Matriorb has been a thing for years and Roxy was forced into the role of attempting to create it by the Condesce, and then chose to continue to try to make it of her own free will.  Jade saving the planets was also an established plot point before hand as Echidna’s request was for Jade to do so. John’s retcon was kind of lazy writing but due to Homestuck being a webcomic Hussie was able to pull that off in part to the nature of the story.  This of course is only my opinion and you are free to yours but I just don’t think it would be strong for the story if Roxy just up and made the trolls’ slabs.