[Vuejs]-Is leaving the <style> section out because I already imported all my styles in the App.vue file a bad practice?

Its fine, its an architecture choice, some prefer a more centralized styling while other use the style tags with or without the scope feature.
Depends also on your needs if styles repeat themselves in many components it makes sense to centralize, whereas a highly specialized style you might want to consider putting in the tag although not a must.