If you add 22 hashtags to your image post, you can certainly add alt text.
One of Mastodon's biggest barriers to entry is the Mastodon HOA, the massive volunteer army of scolds who take it upon themselves to tell strangers that they are doing it wrong. It's like you're waiting in line at the grocery store and someone you've never so much as made eye contact with looks into your cart and decides to enlighten you that meat is murder. They feel better about themselves ...