Yes, edit post & fill the alt text field. Editing posts and setting alt text are standard features of lemmy web interfaces accessed by web browser.
Those downvotes are part of why lemmy has a toxic culture: hostility to web accessibility and basic decency that perpetuates structural inequity. Bluesky and other parts of the fediverse like mastodon are better about promoting web accessibility. Web accessibility is a tangible matter of fairness, equal access, and decency we can all directly provide or deny.