I have been playing piano for about 8 months and I "made" this song when I did not know how to "write or read" music (between month 3 and 6) and when I made it I thought it sounded cool, but now that I have been studying music theory I realized that the song is made with a lot of chords, ostinatos and the left hand is "poor". So, should I be "proud" of that song, or should I try to make it more complex? I know the question may be silly, but what I mean is, can simple compositions be "good"?
First, I completely agree with @alephzero that you should be proud that you made a song, and should make more songs. This is how you get better; if you need to do it, there will be no stopping you anyway.
I don't think that this is a silly question at all; in fact, I would say that it is a good sign that you are asking such questions. It is good to periodically look back on what you have done to assess.
"Should I try to make it more complex?" One of the most difficult things for an artist in any medium to get used to is evaluating their own work (insofar as this is even possible). Try to cultivate some detachment from your work, so that when you look at it, or listen to it, it isn't personal-- it is music. Listen for what is working, and for what isn't working. If you have written a piece that in retrospect seems to not be working, and you feel like there is something there to improve upon, go ahead. If there are ideas that you like and think could be separately developed, take them and start again. But, unless you have specific ideas in mind, I would let it go and start something new. The more things you make and throw away, the easier it is to let go of bad ideas. And there will be plenty of bad ideas.
The song is complex enough. Besides, people like simplicity that's why pop songs that play three/four chords are more popular than jazz that modulates everywhere and has complex chords.
However, this song doesn't seem to go somewhere. It takes alot of different patterns and puts alot of different images in ones head that dont have to do with eachother. Like in beginning I thought of horses racing, then I thought of a kid opening up a present on christmas, then I thought of a drunken organ player, then I thought of a haunted house. See, it doesnt lay out a distinct atmosphere. You need to go a certain direction. Also look into tension and resolve, and having distinct patterns in your song that the listener can identify with. Try to see a "movie" in your head, of what your song is a good soundtrack for. A good way to do this is to add music to a muted youtube video where the video describes the mood of what you're feeling. You can pause the video also to just get an overall feel of what emotions you're trying to evoke.
But good job nonetheless. As Steve Jobs once said "A real artist ships". And as Picasso said "It took me four years to paint like Raphael, but a lifetime to paint like a child." Simplicity is where it's at.