I've known plenty of people like Jessie. She's forward because she's confident, but it's just harmless lighthearted fun trying to get Cloud to stop being so stoic and self-serious like your typical ERA poster (heh). She tries to make everyone around her smile.
Cloud doesn't know how to respond to someone who's so comfortable being herself, because he's unsure of how to be emotionally available to those around him without making himself vulnerable. And that just makes the whole dynamic even cuter.
And while Jessie is goofy, she's also self-aware and, in her quiet moments, deeply introspective, carrying with her the weight of collateral damage from the bombing runs, and always the fear of losing her comatose father to the lifestream. At the end of the day she's still the sweet girl whose parents doted on her, and they mean the world to her.
A great character, and another example of how Remake took the "rough draft" that is the original and embellished it, elevating it to something truly great.