Yes, Math can be self-taught. I am a living example. I went through school, up to 10th grade (junior high school) with only getting minimum passing grades. I was worse at everything except History. That was the only thing that I liked. And even at History I was getting like medium grades, not the highest marks.
Then in the summer vacation between 10th grade and 11th grade I got hit with a hobby of Electronics. I read by mistake an old book, from 60's, that was in my father's book collection, of how to create a radio for short waves that was powered by a potato (the vegetable, not the sarcasm!). And I went down through that rabbit hole. I started to learn electronic components and what they do. That required Physics, which I knew nothing about it - so I started to learn Physics as well. Well, have you ever encountered Physics to be, in practice, applicable without Math? So I started to learn Math all by myself. Between ages of 16 to 18, while I was as Senior in high school I recovered all previous 10 years when I slacked worse than a worm. That allowed me to go to University where I got hit by another hobby - programming. Both these hobbies still stay with me to this day.
So yeah, I would argue not only is possible to learn Math all by yourself, but I'd say that doing so you actually learn it even better then when is forced upon you, even if you're good at it.
Then in the summer vacation between 10th grade and 11th grade I got hit with a hobby of Electronics. I read by mistake an old book, from 60's, that was in my father's book collection, of how to create a radio for short waves that was powered by a potato (the vegetable, not the sarcasm!). And I went down through that rabbit hole. I started to learn electronic components and what they do. That required Physics, which I knew nothing about it - so I started to learn Physics as well. Well, have you ever encountered Physics to be, in practice, applicable without Math? So I started to learn Math all by myself. Between ages of 16 to 18, while I was as Senior in high school I recovered all previous 10 years when I slacked worse than a worm. That allowed me to go to University where I got hit by another hobby - programming. Both these hobbies still stay with me to this day.
So yeah, I would argue not only is possible to learn Math all by yourself, but I'd say that doing so you actually learn it even better then when is forced upon you, even if you're good at it.