No, there are plenty of other farfetched theories most atheists don't subscribe to, lots of other mythical creatures for that matter, from vampires to faeries to the loch ness monster. Most (but not all) atheists are skeptics, and criticism of religion is just one small segment of that. There are in fact entire skeptic magazines and podcasts, often those running them are atheists, that never even mention Gods.
On the same token, there are many who believe in God who don't believe in superstition/mythical creatures. The mistake I see a lot of atheists making is putting God in the same category as these things to the point where they say that God is as believable as a "flying spaghetti monster". It's a silly idea that thinks of God more as a "worldly character" instead of the Creator. It's like finding a book and instead of deducing that there must have been a writer you go into the idea that to believe that an author exist is as believable as a "flying spaghetti monster". It's absolutely ridiculous and flawed.
When you base your conclusions based on "scientific observation" alone and ignore your reason/logic then you are being illogical/unreasonable for you are limiting your own ability of understanding.