Tbh the Cartoony stuff of Gear 5 form actually fits with Monkey D Luffy. Cause One Piece ain't as super serious compared to other Shōnen Manga and Anime. So it's goofiness is part of it's charm. Besides the Manga is better than the Anime Adaptation anyway. If you're expectin' One Piece to be super serious all the time then I suggest you should read and watch somethin' else keep it pushin'. It's breath of fresh air from Naruto And Bleach with their Transformations and stuff.