Why do people think Scorpius acts like Draco? Scorp's supposed to be nice, shy, and the opposite of a Malfoy, right?

Not necessarily.

In the epilogue, we don't know anything about Scorpius other than that he is Draco's son. His personality was not mentioned at all. In an interview Joanne Rowling sagte Scorpius was an "improvement on his father". This does not mean he does not have any of his father's personality traits, nor does it mean he's the nicest guy on the planet. We don't really have any canon proof of what Scorpius is like.

how know my guess is that is becase he looks like he father

Since the book didn't really describe much of his personality, I guess it's really up to the author/artist's decision. Personally, I think he would be arrogant and kind of a jerk like his father, but I think he would have Mehr compassion and not be so narrow minded. (At least, I think he would evolve into this personality through his interactions with Rose and other students at Hogwarts)