Why is Mabel still wearing braces?
She was wearing braces ever since she was 9 years old, normally you only have braces for 1, or in very bad cases 2 years (cases a 9 year old shouldn't have to begin with). But at the end of Gravity Falls, she is already 13, meaning that she could have her braces for at maximum 4 years (but probably more like 3, which is still too much for a growing child)
So my theory is that Mabel never needed braces and just put decorative braces on her teeth as an excuse to have shiny things in her mouth!
But no really, why does she still have those?