<Shrink hat>
If Shizuka's case were a real situation, then most definitely a text-to-speech device would have contributed negatively to her development and well-being. On the other hand, the way the mother was doing it was, at best, giving mixed signals; [to speak normally is good] was mixed with [to speak normally is to punish/harm]. She's already at the age where she (1) understands that her situation is not normal, and (2) has a desire for self-expression. At the very least, rather than consistently scolding her about it (or giving her a text-to-speech device), she ought to be encouraged to be able to speak normally, step-by-step. (Her relationship with Rentarou could have even helped her in this case, but the author, of course, wouldn't want her quirk to go away.)
</Shrink hat>