Ah, Miku...
Wanting Fuutarou to put sunscreen on her = desperate/slutty
He's not even her boyfriend, and their not even dating. It is a desperate move and can even be called slutty. See how Fuutarou just tried to act like he didn't hear her? or he positions himself at the end of the line at the water-slide? well that's because it makes him uncomfortable. Avoidance, this can even be interpreted as a rejection.
Wanting to intrude into Fuutarou and Nino's visit to their manager = needy/clingy
Fuutarou and Nino are going to the hospital. What the hell does Miku think they will do there? have fun? Does she know what a hospital is? Is she Fuutarou's mom that she always needs to keep an eye on Fuutarou?
Thinking that her confession was effective hence her newly found confidence = deluded/misinformed
Has she forgotten how she made a joke/tease out of her own confession? Fuutarou still thinks that she's in love with him regardless of her joke and he still thinks that she's "out of [her]" mind".
I don't know about you guys but being desperate, needy, and deluded are not attractive traits.