This is one of those heavily debated topics. Should you mix your own song? I’ll tell you, when I was an artist I recorded and mixed my own material for years. When I was an artist if you wanted QUALITY mixes you had to go to a "REAL" studio and get someone else to do it for you and that could be pretty expensive. I couldn’t afford that cost so it was DIY for me. That is until I recorded a project that I wanted to release and that is when I went to a professional mix engineer.
Whether a mix is good or not good seems to be subjective, but I do think most mix engineers can agree on a couple of things that make a mix good.
Clarity and balance are two very important things to get right if you choose to mix your songs.
With clarity you need to make sure your overall mix is clear and not muddy or dull sounding. One of the main things that makes mixes sound muddy is too much mid/low mid frequency build up.
When it comes to balance you need to make sure the instruments have the right balance according to the genre. Hip Hop likes to have the drums out front but Rock likes the guitars up front. If you record songs with vocals, making sure the vocals are heard throughout the song is probably the most important thing. Take note of the style you create and make sure to mix accordingly.
I always like the “learn the rules and then break the rules” approach. When it comes to mixing I learn the "Rules" and then I break them. New styles are created when people break the rules.
If you are going to release a single or EP or full length album you should have a budget to hire someone to mix it for you if you are not able to really do a good job. I know people that have released songs and the mixes aren’t good. Now they wish they could take those songs back but they’re out now. They realize their mistake now but it’s too late.
One thing to keep in mind is when you release your song nobody is going to care where you recorded, what DAW you used or who mixed and mastered it. You are going to be compared to major artists with major budgets even though you might only be an indie artist. So, make sure you put your best foot forward.