If I concentrated fully on every game and only played when I knew it would have my full attention and turned the TV off in the background and always played the strategy I thought would give me the highest % chance to win, then maybe I would be level 28 or even 30. But would I actually be a better player?
Yes, clearly.
No, not really clearly at all. Maybe I didn't phrase the example very well, let's try wording the question in a different way.
Hypothetically let's say I started a new Iso account, played 200 games and tried my absolute best and played with no distractions. At the end of the 200 games I have a level of 25. Then I play another 200 games. In these I goof around, try some fun combos and unconventional decks, play while doing other things, and otherwise play sub-optimally. After that my level has dropped to 20. Have I actually got worse at the game? Sure, I am
playing worse, but if I entered a tournament I could play back at my old 25 level again so at that tournament my level really doesn't reflect my ability.
Or to use a non-Dominion example, every year the Williams Sisters turn up to Wimbledon seeded a long way down because they don't really play their best game other than at the majors. They could probably try harder in the tour events and ply more to get a higher seeding for the slams, but it wouldn't mean they were better players at the slams, they would just have a higher ranking.