This is a simple edit of the lyrConf.xml located in Data\Interface, which creates a "Bobblehead" subcategory under the MISC tab of your pipboy (see image). All the bobbleheads in your inventory will appear in that subcategory.
I was honestly surprised to find this wasn't already a feature of DEF_UI, so I added it in myself.
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This is a simple edit of the lyrConf.xml located in Fallout 4\Data\Interface, which creates a "Bobblehead" subcategory under the MISC tab of your pipboy (see image). All the bobbleheads in your inventory will appear in that subcategory.
I was annoyed with all the bobbleheads filling up my inventory screen and was surprised they weren't grouped together by default like the holotapes and magazines were, so I made a small text edit.
If you just want to use the code snippet for the subcategory in your own custom lyrConf.xml, you just need to remember that "SubFilter" number has to be double the amount of your current largest "SubFilter" number.