Capitalize each word in a hashtag (Pascal case). For some words, screen readers might read proper words even if all words are in lowercase but lowercase words might also easily get really weird.
Compare these examples on a screen reader to get a better understanding of what things might sound like for screen reader users.
Even if some word combinations work with all lowercase, it's still better to always use the Pascal case to ensure more of your audience will have a nicer user experience.
At least I also find the Pascal case hashtags a lot easier to read than the ones that are all lowercase.