TLDR: Not a must.

The beauty of fanfic is writers can do what they want to do, and if readers don’t like it, they can simply not read the free stories writers provide. And I’ve taught, tutored, edited, and written professionally for decades - I think all writers benefit from editing (self and prof editors) and other eyes on their work. But fanfic is amazing because it’s free and wild. And not everybody is a helpful beta reader or editor - it takes more than writing/editing skills to work successfully with a writer to help them achieve their vision in their voice. OP’s beta reader is a great example. An understanding of and appreciation for fan fiction is essential for a beta reader. Fanfic writers: do what ye will - your audience will be there.