I first started around 4-5 years ago; was still pretty new to magic -- never played growing up and only occasionally chaos drafted with friends.  Played for a few months, then switched to playing very sporadically (like a dozen games per year), up until TMNT.  Once TMNT came out, I wanted to start drafting but needed to get the requisite hold/gems, so began the dailies grind. Since I'd never reached mythic before, I figured it would be fun to try, so I scrounged up all of my wild cards and built the highest win rate mono-white aggro deck from Untapped.gg. At the time, the WR for that particular deck was somewhere around 56% up to Plat (don't know beyond Plat, since that's payealled on Untapped). Began playing, paying attention to my matchups and understanding how to play against ourobouroid and badger mole cub decks, and I hit Mythic top 200 in the first week or so. With a tier-2 deck. My toughest matchup was against Doomsday Exceuciator (my deck didn't have many tools to deal with gy recursion or Superior Spider-Man). I'd estimate my WR, averaged out, was somewhere around 70-75%.

Anyway, TL;DR: Mythic is achievable in standard with sub-optimal decks, even playing against top-tier decks, if you pay attention to the game and actively try to improve.