Drupal 10.2.0 introduced an updated ui for adding new fields. Aside the label for the field type, you now also have a description, an icon, and the option to group more than one field type together ( ✨ Make field selection less overwhelming by introducing groups Fixed - with the option to provide more information to the field type in a bulleted list on the second step). Problem is that the mermaid diagram field is not following that new pattern and therefore it only displays the label (the screenshot shows the add field in 11.x which uses a dialog modal for that now) :
Aside not being consistent with the rest of the available field types and groups, with just the label alone the field type is also not that self explanatory.
I am not a developer therefore i am not entirely sure but it looks to me that supporting labels, descriptions and the icon requires to move from annotations to attributes? In regard of the micro copy aka the description it would be good to orient to the work in progress over in 📌 [PP1] Refine field descriptions Active . The main rules for the descriptions in the first step of the add a field flow is to avoid the term field and also don't use a period at the end of the description - aside that keep everything as concise as possible in plain english.