There are quite a few editors for Poser library files, some free, some commercial, but all the ones I know of are for a PC, not Mac.

Theoretically there is not much that you can do in these dedicated Poser editors that you can't do in a text editor, but dedicated editors can make some tasks much easier. One of the main problems with using a text editor is the enormous amount of scrolling you need to do to get from one part of the file to another, this is particularly so when editing a figure that has a lot of morphs in it. Another problem is just finding the part of the file you want to edit. Both of those things are much easier in a dedicated Poser editor, and these editors often have special tools to make certain tasks easier.

At a pinch you can get just about get by with a text editor, but some tasks are a lot harder. In a text editor, the Search function is your best friend, as it can help you get to where you want to be within the file, but it takes a considerable amount of experience to get the most out of it.