Let us not collectively try to find reasons to dissuade the OP's daughter from giving FM a go. She likes maths and seems to be good at it.
At this stage, for example, she may not be thinking of a maths degree but her ambitions might shift and focus during sixth form. Most maths depts are interested in maths performance, will be attracted to a candidate offering FM even if most applicants do not have it, and couldn't care less about the third or fourth subjects, except in terms of grade.
My only concern about Art in the mix as a fourth A level would be the potential time sink of that subject. If Art or PE were in a mix of Maths, FM and, say, Economics or Psychology or Geography or a science, then dropping FM would not leave them 'high and dry' as some seem to fear.
My daughter took FM, in a class dominated by boys, and was initially much less confident than the boys. However, she was the tortoise who gradually overtook the hares - so don't overthink that aspect. She was creative but actually chose to pursue that side as a hobby - another option to consider if concerns about Art and PE persist.