Translation:The most stupid comment that I have ever read.
Yes, I think that if they put “ini” at the beginning, that would be better, because some people might wring their head trying to turn this into a sentence and thinking something is missing in their translation.
The question is: Is this a sentence in Indonesian, or just a noun-phrase?
Both work. It's complicated with two-syllable adjectives because some inflect more often than others, but I'm not sure where “stupid” would fall on the continuum. Let's just say that “most stupid” is more emphatic than “stupidest.” http://www.onestopenglish.com/methodology/teaching-tips/ask-the-experts/grammar-questions/grammar-two-syllable-comparatives/146356.article