Is it true, what the girl said?
Sometimes people say things that mean the opposite of what they really want to say.
Sarcasm is a play of words where one uses irony to make fun of another individual. In the present situation, the boy requested the girl to go with him on a picnic outside as the weather was bright and sunny, which is the ideal condition for a picnic. The sudden downpour ruined the initial plan. They had to pack everything up and had to move. In this case, the phrase that the girl used in response to the changed weather condition is representative of an ironic mocking toward the boy, thereby making the statement sarcastic.
Two friends, a girl and a boy, decided to go for an outing because the day was pleasant and sunny. When they reached the spot, the weather conditions changed and the day became cloudy. After seeing this, the girl responded by saying that the day was lovely for an outing.
The change in weather conditions, that is, from sunny to rainy weather is indicative of the fact that the initial conditions of a picnic outside is not available anymore. This means that the plan is ruined and the tone that the girl is using is sarcastic, and not genuine.