The mane 6 are absolutely main characters. I feel starlight is now becoming a main focus of the series as well, so we can consider her one as well. Characters like the CMC, Discord and Trixie are the main supporting characters, as we've seen them more than any really. The others (Zecora, Ember, Throax, and etc) are the secondary supporting characters, which we see on a more time to time basis.