While people being made fun of for being “cishet” is extremely rare and unlikely to happen, you should still help them. Especially if they are an ally, because they would do the same for you. If ANYONE is being bullied for ANY reason you should help them. Especially if they are an ally, because they would do the same for you. If I saw someone being bullied in the LGBTQ community I would help them, because I am an ally, but also because nobody deserves to be bullied for any reason.
Note: If someone is being bullied for being “cishet,” it’s probably not because they are straight and cisgender. It’s just bullies using whatever reason to bully with. Unless they are Transphobic or homophobic, but that is different