Miranda's character isn't even portrayed well but she's still one of the best written characters of ME2 which is funny considering the undeserved praise the characters in ME2 get.
She does something retarded the first time you meet her and the game doesn't allow sheppard to disagree, just accept it. Miranda is just "never wrong". Although she is one of 2 characters that actually has an actual reason related to plot for joining the team (oversee Project Lazarus) which is all you can take. Albeit, only made possible by shoehorning in this new, completely different Cerberus.
Sure she has a change of heart about Cerberus at the end but it's undeserved considering there's nothing leading up to that point to warrant it. She vehemently defends Cerberus until late game. Ass, no matter how great it is, does not excuse that.
On a side note, I still love people seething about Jacob cheating on you if you romance him. It's honestly one of, if not the only, good pieces of writing connecting ME2 and ME3. "She requires a better man than I.." when you inquire about Jacob and Mirands history. Literally the only good, subtle foreshadowing in the entire trilogy that was actually followed up on. Also, he's black. Bioware secretly based.