I think we can answer this question by understanding how the mirrors helped them multiply. As the Rashi you cited states:
כשהיו בעליהן יגיעין מעבודת פרך בשדה היו הולכות ומוליכות להם מאכל ומשתה ומאכילות אותם ונוטלות המראות וכל אחת ואחת רואה עצמה עם בעלה במראה ומשדלתו בדברים אני נאה ממך ומתוך כך מביאות אותם לידי תאוה ונזקקות להם ומתעברות ויולדות שם
When their husbands were worn out from the difficult labor in the field they would go and bring them food and drink, and feed them. And they would take the mirrors, and each one of them would see herself with her husband in the mirror and would seduce him with words: "I am more beautiful than you". And with this they would bring them to a desirous [state] and they would cohabit with them and whey would become pregnant and give birth there.
It seems from this that the mirrors were necessary to stimulate the desire of the men because the men were too worn out from their hard labor to stimulate their own desire. But this was a particular set of circumstances unique to their slavery in Egypt. Once they were free the desire-block that the men had in Egypt would be gone; they would thus presumably desire their wives without the mirror-machinations. Therefore, the mirrors would no longer be necessary in the same way that they had been in Egypt, and it would be fine to donate them for use in the Tabernacle.