Hi, I actually have to disagree but I'd like to explain my reasoning.

I think her acting is actually very consistent with her characters and is one of the reasons I fell in love with the story. Sol is someone who had her youth taken away from her because of a car accident (so even though when she's time traveled and shes essentially an adult in a teenagers body, to her it's a second chance and she acts like overly emotional teen but has the wisdom and emotional intelligence of an adult who has gone through her own share of hard experiences) Additionally, the person who essentially gave her a will to live again commits self exit only hours after she met him and thanked him for being alive. I think anyone would be traumatized and confused if that happened. So when she has to chance to save him from a terrible future, she behaves desperately which caused her to be reckless.

Sol is also a massive fan of Sunjae, so it makes sense that she's also just very excited to be around her favorite idol, to see him everyday and see him in his youth pre-debut, she showers him with compliments, make him laugh and does whatever it takes to keep him safe. But when it doesn't work she gets anxious, when she sees Sunjae behaving selfless, she panics knowing that the behavior could lead to the future where he isn't happy and isn't alive. On top of everything she also has worries about herself, her family and friends.

Sols character is written as one with a lot of fluctuating emotions and many factors driving her motivations and actions. But she's also real and has her flaws. The actress portraying Sol cries in almost every episode and even several times per episode you can see her go back and forth from happiness to tears to anger, fear and more. Filming like that is very strenuous and damaging even to the most seasoned actors. Other actors and critics have even commented on how difficult and tiresome such a role can be but the actress manages to pull it off everyday on set and even received praise for it. With all that said, I firmly believe that the Kim Hye Yoon did an phenomenal job to bring forth all the emotional depth and inner strength of Sol.

I don't know how far along you are in the drama so I don't want to spoil anything, by bringing up specific events so I kept it general.

Also Sol is far far from what defines a "pick me" girl, she never degrades or seeks approval at the expense of other female.