Yep. It's very important.
If you don't mix and master your tracks, your music might sound:
1) Your music, and certain sounds and instruments in your track, might sound too loud and have tons of clipping. aka the (red zone
2) Your music might be too soft, for listeners to hear.
3) Your music and synth sounds, might sound muddy, blurry, very low sounding and not clear.
4) Certain synth sounds and instruments, might get drowned out, by other louder sounds.
5) Certain sounds, might sound dull and weak. Especially when louder sounds are playing.
6) Certain frequencies in some synth sounds, might hurt the listeners ears.
7) Some synth sounds etc. might sound thin and bland and not full/wide. Making you track/sounds kinda thin and bland.
8 ) Your music might sound like noisy noise, to some listeners. And they might refuse to listen to it.
And a ton of other things, that will take too long to write.
Mixing and mastering is not that super hard. Well,....sort of.
And sometimes, it depends on the type of music project, you're working on.
When it comes to mixing and mastering, learn what you can learn now, as time passes faster
than we think.
If you have like a basic idea, of how to mix and master music tracks, that alone is good enough
to help you later on, with music tracks, that have lots of different sounds and instruments.