There is a recurring trope seen in mainstream media that a child or person with Autism is excellent at Math, possibly even possessing savant skills.

Is this really true? Are kids and adults with Autism better than the general populace at Math?
What is Autism?
Autism is a Neurodiverse condition, characterized by repetitive, restricted and inflexible patterns of behaviour, interests and activities. It also generally manifests as difficulties in social interaction and communication.
Autism is a spectrum, meaning it can manifest in different ways and in different severities in different people.
Real world Autistic Math Prodigies
There are, indeed, some real-world, Autistic, Math prodigies. A few of the famous recent ones are below -
Daniel Lightwing
Daniel Lightwing is the real deal from which the movie "X + Y" (or rather the book on which the movie was based) was inspired. Daniel is on the Autism Spectrum and has been diagnosed with Asperger's syndrome.
Learning to read early and having a voracious appetite for learning, he was even bullied by a teacher before he found his love for Math. He eventually represented UK in the international Math Olympiad. He worked at Google as well as several gambling-related firms, before cofounding Castella Research.
Daniel is quoted as saying (about Asperger's) -
"I wouldn't call it a disability. When you have Asperger's you are putting on a mask and trying to pretend you are normal but what you are thinking is not normal."
Jacob Barnett
Jacob Barnett is an example of what an autistic child with gifted math skills could look like.

At the age of 2, he was diagnosed with autism, and was told he wouldn't be able to read, write or learn anything (including tying shoe laces). He recounts some of that in his really fun TED talk here -
However, he was later found to have an IQ of 170 in Math - which is the highest possible. At just 15 years old, he joined the Perimeter institute in Waterloo, to pursue research in theoretical Physics.
Sawyer
This (then) 4 year old shows how good he is with numbers (and can even read well above what you expect at this age!)
Are All Autistic Kids Good At Math?
While we can all agree that there are some kids whose Autism really propels their Math skills higher, research suggests that the opposite is likely for a lot more kids.
In other words, some kids with Autism might be mathematically gifted - and they can be an inspiration to all of us. Indeed, this means if your child is autistic, do not consider it a natural outcome that they are poor at Math. It is better if you can evaluate separately how adept they seem to be at learning Math skills, independent of their other autism-related symptoms.
However, a majority of them could struggle at Math without additional support and can be behind other kids. Difficulties in Executive functioning can be the main driver of such a struggle.
Having unrealistic expectations placed on kids with autism who are not gifted at Math (such as "all autistic kids are great at Math") can be harmful if it leads the child to self-doubt or to receive lesser support than is needed by them.
How can Autistic Kids be Supported for Math?
Kids with Autism are generally behind in social skills, which hampers a lot of learning techniques that rely on communication. On the other hand, self-paced or self-consumed learning can work well for them, especially if it's at the right level and especially if it can tap into visual learning that kids with autism seem to be good at.
Multisensory learning can also be useful for them.
Research suggests that math games could be more appealing to kids with Autism, compared to learning in a classroom setting. This makes sense, given that kids with Autism do like to play games. Math games like Monster Math, which are specifically designed to address Neurodivergent needs, can help Kids with Autism learn Math visually and in a fun, self-paced way. This also allows the child to learn at their level, rather than being forced into a particular grade level.
If you have a child who is Autistic and either great at Math or needs some support with it, do try Monster Math to help build their Math Fact fluency and also develop a fun relationship with Math!