Philippe de France sounds more historical than Gargamel Capet.
Blame Paradox for merging the name of the dynasty with the character names (there's a wishlist somewhere, why don't people go there and ask for dynastic/character names split ?). Academic names are just that, they are not last names and don't fit characters : you can't call an individual in the game "Philippe Capetians" it sounds ridiculous just like "Guillem Bosonid" or "Alfonso Jimena".
Now Capet specifically is semi offensive to french people who know history because (and it's the last I repeat it before I quote Le Goff on that) its only other use in history besides being the individual title/nickname of Hugues Capet was to ridicule the king at the french revolution (the story of the widow Capet certainly you read that in your history book about the capetians ?). Once again Capetians/Capétiens is fine, it's just gibberish when used as a last name and imply ever other french dynasty should be renamed to the academic adjective, or all capetian cadet branches to Capétiens-Bourgogne, Capétiens-Valois, Capétiens-Vermandois and the like. Well I suppose you don't find anything wrong with "Philippe Capétiens-Bourgogne" as a character name anyway.
I'm a bit tired of arguing now, I'd rather work on the mod than spending time answering the same question again and again. Maybe you'll launch the next version of the mod and have all the families named Guillelmides, Robertiens, Capétiens, Ramnulfides and the like and I'll hear the opposite argument from the same people for a change.
Don't mind the complainers Solo, I was going to respond to that post as well, since he linked your explanation right before asking the same question you had just answered but it was 4am (my time) and didn't want to bother. I say stick with it and ignore the questions. I've found that overhaul mods get tons of pages of questions of this and that and it's hard to filter out all the 'noise' when the pages of responses add up. Keep up the great work