I have noticed a lot of people seem to have a problem of Starlight Glimmer's redemption as too quick. I don't think that the case.
Remember during the song Friends Are Always There For You? During the monologue sequence, where Starlight is socializing with Princess Twilight and her friends (plus apologizing with residents of Our Town). I am pretty sure it took time for Starlight to actually trust Twilight and her friends.
Here is a fan fiction of the apology to the residents of Our Town by Starlight = http://www.fimfiction.net/story/303080/in-their-town