Lossless formats such as WAV and FLAC offer higher quality sound reproduction than MP3, as they retain more of the original details and nuances of a recording, without lossy file compression.
MP3 is a compressed, lossy audio format--which means that some of the audio data is removed to make the file size smaller. While MP3 is a popular and widely-used audio format, it cannot match the sound quality of lossless formats.
DistroKid accepts many file formats, including WAV, FLAC and MP3. But it should be noted that an artist should only upload music in MP3 format, if the artist is comfortable with the sound quality of their MP3 and wishes to use it for distribution.
Ultimately, it's up to the artist to decide which format to use for their music. But they should be aware that using a lossless format such as WAV or FLAC can provide the highest quality audio and ensure that their music meets the standards of discerning listeners.