Yes and no. If you are using the duty finder, the game will automatically pair you with companions also on the lookout for a party. If you choose a dungeon lower than your current level, you will synch to its level. However, you can tweak around the duty finder settings to join one with the Undersized Party. This option lets you enter the dungeon with your current party - or solo - and won't synch your level or item levels. So if you join a level 50 dungeon with your level 90 character, you will bulldoze all enemies and bosses. While you won't earn any exp or items this way, you can still complete dungeon-related quests and Wondrous Tails tiles.