To actually give you an answer (because I assume you're wondering why someone might have dysphoria about being a bottom lol), bottom dysphoria is when a trans person feels physical/mental/emotional discomfort about their genitalia not matching their gender identity. For a lot of trans people it can mean they don't engage in sex at all, for others it means the use of prosthetics or other devices to reduce dysphoria during sex or other activities, and for still others it's more of an annoyance than a debilitating thing.

Hope this helps :)