Does a .wav file created from a .mp3 file have better quality than the .mp3 file itself?
MP3 is a lossy compression format, so encoding an uncompressed lossless WAV file to MP3 will lose some quality in the encoding process.

Decoding a MP3 file back to WAV file has at most the same quality as the MP3 file has, it is just as good or bad as the contents in the MP3 file was, it is not in any way different to decode it for saving into a file or decoding it for playback. Most importantly, you don't get the lost quality back when decoding, and basically the result actually depends on the quality of the decoder as well.