Oh damn, this post is old now!
It’s a children’s word. It’s exactly the same as how children in English call a train a ‘choo choo’. And then English people thought it sounded a combination of cute and stupid, which tied in with the whole ‘Welsh isn’t a real/valid language’ thing, and from there it continued because no one listens to Welsh people (I’ve literally had more than one conversation where I’ve explained all this and an English person has looked me in the eye and said “Well I prefer it so I’m going to keep saying it’s popty ping, tee hee”. I also vividly remember an English person on Tumblr - who is otherwise lovely - responding to me on this topic to explain to me that if people start using a word then it becomes a valid word, so actually popty ping now IS the Welsh word for microwave, and I had to explain back that no one is using it outside of nurseries and English people, now please will you actually believe us.)
Basically… imagine if the internet decided that “the English word for a set of carriages pulled on rails by an engine is a Choo Choo” and then either patronisingly simpered about it or sneered about how your language should therefore be wiped out about it and you have the issue
And as a final note, the whole thing is extra ridiculous because Welsh has three different terms for ‘jellyfish’ and every one of them is way more charming and batshit than popty ping on its best day AND are the actual genuine terms, so it’s a missed trick