When you're posting hashtags that contain multiple words, it's a good idea to capitalise the first letter of each word. This helps blind people who use screen reader software to know what the hashtag actually says.
For example #DogsOfMastodon is much easier for screen reader users than #dogsofmastodon, even though they're technically the same tag.
This kind of capitalisation is usually known as "CamelCase". CamelCase doesn't just help blind people, it also makes hashtags clearer for sighted people too!
Using CamelCase won't affect your post's visibility (because searches ignore capitalisation) but it will improve the post's accessibility, so you'll get more people reading it.
Thank you to @martyn for raising this topic 👍