Hello Noa. How are you?
I have completed Ultra Sun (Including the RR story) with a Grass Monotype team. The star of my team was an adorable Lilligant I acquired from the GTS as a Petilil. She has a Naughty nature. I thought it would add to the challenge by using a random Petilil instead of breeding a near perfect one on Sun and transferring her in.
I think Chlorophyll might be the preferred ability competitively but Own Tempo is a great ability in game. Sleep Powder, Quiver Dance and Petal Dance is a great combination and helped me greatly in my Playthrough. I have her Dream Eater too in case I ran into a Poison type.
The rest of my team was Tsareena, Decidueye, Venusaur, Shiny Shiftry and Totem Lurantis.
I have now Grass Monotyped all regions except Hoenn. :) Incidentally the only other time I was able to use Lilligant in a Monotype (Unova) she was also very strong.