Matchmaking is based only on MMR. See How Competitive Skill Rating Works (Season 12) → Matchmaking.
The average person you are placed with has the same ability to win games as you do. That is, he is just as good as you. If your aim is indeed master level, then there are some other weaknesses in your play (such as game sense, attitude, ult economy, etc…). Without seeing your profile and a VOD of a narrow loss of yours, I can’t speculate with any degree of accuracy.