How do I get better at CS:GO?

Doesn't really matter what gamemode you play. Just, the more you play the game the better.
The best way to improve a skill is to do said skill over and over again, trying to improve yourself in every way possible.
As long as you're playing the game, you can play whatever. Deathmatch is ok, but you won't have much chance practicing pistol skills against people with AKs and M4s. It's better to play Casuals or Competitives for that. The more games you play the better your rank will become if you derank, that's OK too. You'll play with people more suited for yourself then, and you'll have more fun. That's what games are all about.