In the hots casual 'lobby' aka quick match, you often don't have enough players of similar skill level, so the match maker tries to create balanced teams with quite a large mmr spread. There are many ways this goes wrong, but the end result is your winrate tends towards 50% because that's what the match maker is trying to do. It performs very badly at this and so you can have big streaks and bad quality matches for an extended period of time.

I had a week straight of terrible matches that were almost always lost at the loading screen. There's only so much you can do if the map is bad for the heroes on your team or your team has a terrible composition while the enemy team has a literal meta S grade composition. It didn't help that the match maker for whatever reason decided that I was good enough to take on two or more diamonds with the rest gold vs my team of me and silver/bronzes. One of those losses was a master with two diamonds and the rest silver vs me and some gold/silver players. It's hard to get people to play well when the loading screen shows a massive disparity in game skill.

Nearly every one of those matches I ended the game with the best stats and still lost horribly.

My favorite though were the team mates that threw at the start of the match or when my team would decide they would rather argue than play the game. Such a toxic experience of a game. Having one amazing match of great fun and teamplay just wasn't worth the 2-5 matches of absolute toxicity.

Also, I don't mean to come across more negatively than I actually feel. The game can be frustrating at times, and there could be improvements made to reduce those frustrations.

But overall, it's still possible to have a lot of fun and great matches enough of the time that it is still worth playing.

I'm around low diamond, which is definitely my skill ceiling. It relatively easy for me to assess higher diamonds and masters. It's also easy to asses plat players. Anything below plat looks very similar to me.