How the forget
Does a tiny pegasus
Beat an indestructible futuristic robot
:l
Because
1) Death Battle is absolute trash
2) Death Battle is a popularity contest (see 1). If the pony lost, the bronies would get their knickers knotted beyond recognition and storm the gates of whoever made that video.
Granted, there are some logical ways that Dash could have won, by say, exploiting starscream's horrible accuracy with the null-ray and getting him to hit something like a volatile tank of explodium. But there is absolutely no logical outcome in which a miniature horse could best an armored construct in physical combat. See 1 and 2.