Mars One was a questionable organization that received investments by claiming it would use it to land humans on Mars. It was not a credible company, and many consider it was a scam to take money from investors and participants.
Mars One had a lot of media attention, but it was dissolved without any achievement. Flat-Earthers use it to discredit space travel. In reality, the company was not credible in the first place, and practically nobody in the aerospace industry thought Mars One was serious in their objective.