how is that a "plot problem"?
It's like saying that a henchman is more powerful than the villain that hired him. Apocalypse was given his powers by the Celestials and it's been well established that he isn't as powerful as any one of them. Internal consistency is a very important thing in storytelling, and stories can begin to fall apart if a writer doesn't maintain it.
From my point of view when it comes to this axe, now that we've seen it kill a Celestial, I fear I'm going to be taken right out of the story every time it's used. If it split a Celestial's armour, why didn't it destroy that wall it hit when that hero ducked? If it split a Celstial's armour, why didn't it put a huge crater in the pavement after that hero moved out of the way? These sort of questions are going to come up quite often in my mind in the next few issues. I fear it's going to get really annoying and distracting.
In short, I hold firmly to the thought that showing the axe doing that this soon into the story was a really bad idea.