I'm gonna have to disagree with Cor - A good mix cannot save a bad composition and a bad mix usually won't sabotage an amazing song, but the mixdown is one of the most integral parts of a song's creation. The mixdown is where you decide what elements are going to be at the forefront, which elements are going to be pushed into the background, and overall just shape the feeling of the track.
Sure, you can pay someone to mix your track - professionals do it all the time. But you also have to deal with revisions and all the money you'll be spending on the services. There are some viable arguments for and against having someone else master your music, but even then I feel as though it's something people are making out to be much more complex and mysterious than it actually is.
Learning to mix and learning to master are not difficult skills and they are essential skills to have as a music producer, so you shouldn't encourage people to disregard them. Investing in a solid pair of reference headphones (or, even better, a pair of studio monitors) will go a long way to improving the quality of your music.