Rowlet's evolution chain has a nicely even distribution of stats; you could go a bunch of different ways. If you don't use T.M.s, there's literally no question: adamant (+attack, -special attack) is the way to go. In-game, attacking power is the most important thing you can have. Plus, he doesn't learn any special moves in Sun/Moon. The special attack moves he learns in the ultra versions aren't that powerful, and decideye doesn't have access to a lot of good special T.M.s in either game.
If you can't get adamant, you could go for a jolly nature, which pluses his speed, and decreases special attack. It would give him the upper hand earlier in the game.