If I can give an advice, Derpibooru’s users tend to upvote the characters that know better. I.e., the characters from the show.
Didn’t you notice that the first 5 most upvoted pictures of yours are all regular characters?
Your Derpy is good. A score of 50 for a beginner is good too.
If people don’t comment, is that because there’s little to comment.
Draw cuteness. Draw funny situations. Heck, even draw shippings.
As your last resource you may even draw porn (I don’t recomment to do it if you don’t practice a bit first, though).
If you want people to comment, give them something to write a comment about (without trying to start shit, of course). Even if it’s something like “Ew, Fluttermac!”
If you really want to draw an OC, be sure to be damn consistent about it. Create a story, create interactions, don’t create Mary Sue’s, please nothing overpowered, make connections and references with other regular characters or popular OCs. And, of course, don’t be boring about it.