I'm sure that somewhere out there, there's a Sonamy fanfic in which Sally is a prominent character and not portrayed as either a whiny, OOC brat or a "Regina George meets The Evil Queen"-esque villain. Sadly, I have never been able to find a fic that matches that description. (Seriously, if you know about one, send me a link! I am dying to read it!)
Obviously, this is an exaggeration, but it still holds true to the cliche. If anything, this portrayal of Sally's personality is closer to the character of Alicia (The ANTI-Sally). This version is catty, possessive, and generally a desperate slut. The actual Sally is courageous, smart, and honorable- and if Sonic were into Amy, she certainly wouldn't try to scheme her way to breaking them up.
If you hate Sally, fine. You are as much entitled to your opinion as I am to mine. But writing Sally as the villain simply to help speed along another ship is just low, and an insult to her character. If you write Sonamy, don't fall into this overused trope.