I have never used the SoundCloud export feature so I don’t know what it really sends out, but I can tell you that if you want the best sounding result on SoundCloud, don’t use mp3. Regardless of what you upload the SoundCloud, it will be streamed at a very low bitrate (64kbps Opus I believe).
So, if you upload an mp3, it will get reduced again to a really poor quality stream. Transcoding in other words.
If you can upload a 24-bit/44.1k WAV with a full dB of peak headroom, that will typically be the least worst sounding when SoundCloud streams it to the listener.