the thought of "MDY is actually/should be MTY" was present and around when the anime was aired,
and I think that thought is wrong
I think MDY is correct because, as it is written in that manga there, it is just from マジ で ヤバイ or Maji De Yabai,
so M-D-Y
I believe it is a short form of "maji de kore yabai" which means the same
sometimes shortened further so it's just マジヤバイ (Maji Yabai)
and it is a word that
Japanese people said both in
positive and negative situations
though, I don't believe Japanese people actually say the romaji MDY in real life as seen in the anime
that was so weird