>>13854329Laplus is supposed to be the leader of HoloX and as such demands respect from her genmates and viewers alike. The issue, is that Laplus is very cute and childlike, and thus naturally, does not demand respect from her older genmates or viewers. This is phase 1 of the joke.
As part of Laplus demanding respect as a lord of darkness, she requests that all genmates and viewers address her with the greeting "Yes, My Dark." This is a lot of english letters for JOPs to type, so ot was quickly abbreviated to "YMD", which annoys Laplus as she views it as disrespectful to her dark authority. The problem again, is that Laplus is very cute, and so her viewers quickly come to enjoy teasing her by using YMD instead of Yes My Dark. This is phase 2.
Japanese fans realize that YMD when spelled out shares a lot of letters with the common Japanese surname "Yamada", and in a further attempt to simplify the phrase, begin responding to her call for respect with "Yamada". This leads to Laplus thinking that the viewers are calling HER Yamada, which through thr power of self fulfilling prophecy, leads to her viewers and genmates actually calling her Yamada, teasingly.
So now we have a situation where the cute loli demon lord demands recognition from her followers, who commonly respond with a patronizing "aww look, its Yamada chan. " "Little Yamada chan is so cute!" which is the complete opposite response her character wants.
Tl;dr: Yamada chan is very cute and its very fun to tease her.