All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is no blame on the one who supplicates Allaah The Almighty to give him a good voice. As for the issue of Riyaa' (showing off), it is not necessarily related to the supplication of getting a good voice. One may ask Allaah The Almighty with that supplication while having a good intention such as intending that a good voice will help him contemplate the Noble Quran and that a good voice will be a means of benefiting people from his recitation. On the other hand, one may ask Allaah with that supplication while intending to get the praise of the people because of his good voice. So, there is no certain relation between asking for a good voice and showing off.
However, what a person should do is to concentrate on contemplating and understanding the meanings of the Quran and to invoke Allaah The Almighty to bestow that gift on him.
For more benefit, please refer to Fatwa 89655.
Allaah Knows best.