I agree with Mariana. In the formal way of writing, we never write it in katakana, however, in the casual way, for example, text messages between friends, we sometimes use katakana. In the casual way, there isn't specific rules, we can use whatever we like. So, I think it might confuse the learners. The same happens in English and other languages, doesn't it? Like "luv ya", "2 you", "4 you", etc...

I might be wrong, but katakana is sometimes used to create emphasis in a word or sentence. In this case I suppose it's just to be more informal than "最低".

People over about 30 years old tend to write anything in katakana to emphasize the words. So I think this way is old expression.