How would average Americans know how they looked like?
At this time, newspapers were printed up to three times a day, with morning, midday and evening editions. They needed to tell stories, lenghty ones, stories that "average american people" would read and read again. Even if they had to twist events and truth, they wrote again and again to sell paper and ink, not quality. Tabloïds still do the same nowadays. And criminals are still very popular in the USA today.
There are few couples who have made headlines in quite the same way as Bonnie Parker and Clyde Barrow. The two criminals are known for a series of bank robberies, murders, and kidnappings that took place between 1932 and 1934, the height of the Great Depression. Library of Congress
They became one of the first outlaw media stars after some photos of them fooling around with guns were found by police, and the myth-making machine began to work its transformative magic. Soon fame would turn sour and their lives end in a bloody police ambush, but their dramatic and untimely end would only add luster to their legend. Biography
And why would people adore them knowing they had murdered policemen as well?
As they were famous, hitting the headlines many many times, people started to view them as "heroes against the machine".
The legendary quality of Barrow’s and Parker’s careers is not difficult to understand, given the extreme desperation of the times. Their crime spree occurred at the height of the Great Depression, which hit particularly hard in states such as Oklahoma. Several bank robbers during this period became famous as “Robin Hood” figures who struck back against the banks, which many people viewed as oppressive. Britannica
Until the true nature of their crimes was revealed, they remained "heroes". Then, it slowly switched to rejection. Even after that, for some people, like serial-killers, they still had fans.
A complete digest of their story at The crime museum.