No, being interested in or even obsessed with aliens does not automatically indicate that someone is autistic. Interests and hobbies can vary widely among individuals, and being fascinated by aliens or any other topic is a common and normal part of human diversity.
Autism is a neurodevelopmental condition characterized by differences in social communication and interaction, as well as restricted and repetitive patterns of behavior, interests, or activities. While some individuals on the autism spectrum may have intense interests in specific subjects (often referred to as "special interests"), having an interest in aliens alone does not imply that someone is autistic.
People from all walks of life and various backgrounds can have a strong interest in aliens or any other topic, and it's essential not to jump to conclusions or make assumptions about an individual's neurodevelopmental status based on their interests.
If you or someone you know is concerned about autism or any other neurodevelopmental condition, it's best to consult with a qualified healthcare professional or specialist for a proper evaluation and diagnosis.