This is an awkward question with a not so simple answer. Since autism isn’t diagnosed until a child turns two or later when noticeable delays appear, babies aren’t autistic, yet. Babies, by eight weeks, try to mimic what they see in parents’ faces and work their facial muscles. By four months, they are smiling on their own, but since it’s not always in response to something that might be funny, it’s unconfirmed if they are just reflexively smiling. By six months, the laughter a baby exhibits is directly related to things they do find funny, like their mother’s smile or tickling toes.
The issue of smiling or not comes into play in a diagnosis of autism when a child does not make eye contact or attempt to look and copy parental facial expressions. When the smiling isn’t for the purpose of a social connection to others, it’s worth taking note. However, parents shouldn’t be concerned unless it precedes a long stream of other developmental oddities, and then the pediatrician should be notified at a regular healthy baby checkup.