I left a comment but it didn't go through for some reason. Here it is again:
Sorry for the late reply. I don't know what a style guideline would entail, exactly. The only thing I can think of is in the naming of communities. And that can be explained in one sentence.
Communities should be named in an easy to write way, and easy to read. For example, lunar_punk should be LunarPunk. Say there is a community called word1word2. That should be Word1Word2. It is easy to read, thanks to the capitalisation of the words. It is also easy to write, since there are no underscores, just normal characters and one doesn't even need to capitalise the words for it to be a valid URL.
I don't use mastodon. I think my naming suggestion still works best for tagging on mastodon though. It is still more visually appealing.
But ya, I still see naming on Lemmy as one of the big hurdles towards getting a bigger userbase. There is just some annoying friction that pisses me and I am sure it pisses newcomers even more. Instances with names such as lemmy.dbzer0.com.
Another thing I hate is that the UIs don't show the true name of the community but their alias. For example, we have a community called climate here on slrpnk. However, UIs (both mobile and desktop), show the name "climate - truthful information bla bla"... I find that to be horrible UI. It should show that the post is coming from "climate@slrpnk.net". I hate hate hate that.