I really love Allison, and I agree that she's not some psycho bitch, BUT I can see how some people might think that - especially if the player isn't especially interested in her while she's pursuing Ian regardless. She's clearly had feelings for Ian for a long time (apparently she liked him since school), and loving someone for a long time, then getting an opportunity to actually be with them -- them both being single -- can naturally make a person a bit desperate to finally see it happen. I think Eva got Allison's desperation pretty spot on to reality - wanting him to pay attention to her first and foremost all the time (how she always butts in so he'll talk to her), how she tries several times to hook up with him even after she's screwing Jeremy, etc.
Regardless of if you like her or not, he's a pretty real character, and you gotta admire her dedication to going after what she wants.
I also hope, like you, that Allison's personality continues to get fleshed out and that she and Ian connect beyond just pure sex - maybe during their holiday together. I'm looking forward to that content.