Female Dwarves have beards in DND when the Dungeon Master (DM) and/or Players decide that they will have beards. While it is true that many female dwarves in D&D do not have beards, there are some that do. This is usually due to the fact that the Dungeon Master or players have decided that they will have beards.

Traditional Female Dwarves grow out their beards and look like males when in armor or thick clothing. In their society, a dwarf is a a dwarf. Modern Female Dwarves shave their beards and grow out long hair. Female and male dwarves have different gender roles and the hair symbolizes it.

There's no direct line from Tolkien about female Dwarves actually having beards, but stating that they look like male Dwarves, and male Dwarves all have beards, heavily implies that they do have beards.