First of all, if you don't already have the InSimenator, you should get that... it will make this process SO much easier. I recommend the DEST version of it, but read through and decide which one's right for you. Once you have this in your game, click anywhere on your lot (on the ground), and choose "Spawn...", then choose "personality adjuster". An urn will appear... this is the personality adjuster. Click on it and go to "Personality 0%"... set "My Niceness" to 0%. VOILA! Your Sim will soon alienate him/herself from everyone they know!
RECAP: Click on any floor tile--> "Spawn"--> "Personality Adjuster"-->"Personality 0%"-->"My Niceness"
Now... if you're talking in terms of physical unattractiveness, just use the plastic surgery career reward. If they don't have platinum aspiration, they'll come out deformed anyway. If they do, you can decide which features you'd like to change to make the sim as physically unattractive as you want

Sorry for the LONG response, but I hope it helps!