In order to get my girlfriend to join Stack Exchange, I sometimes write questions using her account

This isn't going to help her "join" stack exchange any more than you reading books for her will help her join the library.

Even ignoring the artificial reputation you're creating by upvoting your own questions after making them, the fact that you are writing the questions means she's already receiving reputation you believe she wouldn't receive posing her own questions.

Further, she doesn't get the experience to learn how to write good questions if you're doing the work for her.

Ultimately it does her no good. When she does write her own questions she may end up doing badly and leaving even after all your work.

I strongly suggest that instead you sit near her when she's writing questions and critique them, teaching her how best to ask questions that get answered. It will be much more a benefit to her than simply writing them for her.

In order to get my girlfriend to join Stack Exchange, I sometimes write questions using her account

This isn't going to help her "join" Stack Exchange any more than you reading books for her will help her join the library.

Even ignoring the artificial reputation you're creating by upvoting your own questions after making them, the fact that you are writing the questions means she's already receiving reputation you believe she wouldn't receive posing her own questions.

Further, she doesn't get the experience to learn how to write good questions if you're doing the work for her.

Ultimately it does her no good. When she does write her own questions she may end up doing badly and leaving even after all your work.

I strongly suggest that instead you sit near her when she's writing questions and critique them, teaching her how best to ask questions that get answered. It will be much more a benefit to her than simply writing them for her.