Ok a couple of things spring to mind.
Mimi is a drug addict, as revealed in the song 'light my candle'. It could be said that a drug addict would not have full concideration to the consequences of her actions.
Roger recognises something in her that reminds him of his previous partner, and also somethings in her life that he can relate to.
However, there is one thing that your question does miss. If they were going to have sex why wouldn't they use condoms, roger is well aware of his status, and is in control of his HIV as seen by the segment where everyone pauses for medication. Even if you assume Mimi is not able to make that decision - Which I don't, Roger is.
You ask if Mimi is a bad person. No, she is lonely, looking for love or partnership, a connection, as is Roger. We all make mistakes and bad decisions when looking for love, or seeking that someone special.