"You're so cute" IS the a sentence in the present simple. Every sentence has to have a verb. 'You so cute' is not proper English. It's baby-talk or pidgin English. 'You are so cute' or the shortened form 'You're so cute', is the present simple form of the verb 'to be' (I am, you are, he/she/it is).

"You are cute" means that you are cute right now and in general (always). "You were cute" means you were cute in the past (but not necessarily now). If you want to say that someone is cute this very moment then you can use "you are cute". If you want to specify when exactly you have found that person cute then you can always add the time: "You are so cute right now." "You are cute when you do that." "You are always cute." "You are cute sometimes."