I don't know how many cakes I've made over the years. None of them have failed, including the very first ones years and years ago. Who knows why mangaka insist it's so difficult. Of course there are really complex designs and structures, in many ways, but why would an amateur attempt to make such?
Like in this one-shot, it's not really a question of difficulty, I don't think I've ever seen a mangaka make basic cooking seem actually difficult, it's just that some characters are so airheaded and/or stubborn that they completely ignore basic instructions for the recipes they themselves set out to make. Takamo didn't sift the flour, didn't seem to measure it, and burnt the cake, something that only happens if you don't properly set the oven and pull it out in time. If Natori did his job of watching over her properly and stopped her whenever she messed up an instruction, she could've made a basic cake herself. And if he wanted to after that, he could've done the difficult part and decorated it for her.
It's a trope you see a lot in anime/manga meant to make a girl seem clumsy and endearing and to set up the reoccurring joke that other people have to eat their bad cooking. I'm pretty apathetic to it, tho it usually makes me like those characters less. Also, characters like that often try to make complex stuff either as a setup a gag or the plot, like it is here, or cuz the author wants to make them seem even more endearing for being bad at something but still trying their best, again like Takamo is doing here.