I Fully disagree to literally translate this expression into English! Why not eg ' Eating custard is no big deal' ?
I believe they left it with the literal translation as ¨geen kunst¨ & ¨not an art¨ seem to be a shared idiom between English and Dutch, also I could use a sentence ¨doing something is not an art¨ to imply that the person is being to cautious or being a perfectionist in the way they are doing said task, whereas you wouldn't be able to use ¨is not difficult¨ in this situation.
Story time: Once, during my visit in the Netherlands, which took place couple of years ago, I found a box of vla in our host's fridge, and my mid-European brain thought to itself: "Wow, I didn't know they call milk vla in Dutch."
I think you can imagine my face after I tried to dump it in my drink.
in the beginning of this exercise, vla was pie and now it is not accepted here