It's hard to say for sure if it's extremely popular. There are so many different types of Naruto fanfictions out there. However, some fans might be attracted to this concept as it shows a different aspect of Naruto's character. It could have a niche following among those who like to see characters in unexpected roles.
Well, 'naruto is a flirt' in a fanfiction might mean that in this fictional story, Naruto is depicted as someone who flirts a lot. Maybe the author wants to explore a different side of Naruto's character, showing him being more charming and flirtatious with other characters.
Some fanfictions might have a more mature or experienced demigod flirting with Annabeth. Maybe a guy who has been on many more quests than her and uses his stories and adventures to impress her. Or it could be a character who is known for his good looks and confidence, like a son of Aphrodite, who tries to use his natural charm to flirt with Annabeth.
It was not an idiom to say that fireworks were seductive. The fireworks were usually an adjective phrase used to describe the lively and happy atmosphere of a fireworks display or a festive atmosphere. The 'fireworks' in this phrase referred to fireworks and firecrackers, while the 'flirtatious' meant to stir up people's hearts. Therefore,"fireworks" could be understood as describing a lively and happy scene rather than an idiom or phrase.
One interesting aspect could be the character development of Harry as a flirt. It might show a different side of his personality that we don't often see in the original stories. Maybe he uses his charm in unique ways to interact with other characters.
Once there was a wife named Lily. She was at a neighborhood party. She noticed a new neighbor, Tom. She playfully flirted by complimenting his taste in music when she saw his band T - shirt. Then she winked at him and said she hoped to hear his favorite tunes at the next block party. It was all in good fun, just a friendly and flirty exchange.