Overall, I would say that no, Mega Meganium is not viable in Legends: Z-A Ranked Battles.
Armarouge is currently dominant and this is a suicidal matchup for Meganium. Because the meta also revolves around prominent Dragon-types (Garchomp, Rayquaza) and Steel-types (Melmetal, Gholdengo, Magearna, Skarmory), there is also an abundance of powerful Ice (Baxcalibur, Glaceon, Kyogre and others with Ice Beam) and Fire (Volcanion, Charizard) users. This makes it very hard for Meganium to survive.
Another way to look at it: what is Meganium supposed to be good against? Grass is one of the worse offensive types due to the aforementioned common threats. Due to the base form's poor offensive stats and lack of Fairy typing, Meganium's utility is extremely limited until it's able to Mega Evolve to make the best out of Dazzling Gleam, which is still relatively underpowered. Overall, it's not worth wasting your Mega Power on since you're putting a lot of work in just to get mediocre results.
If you want to use a bulky Fairy-type, go for Xerneas or Clefable. If you're set on a Grass-type, Rotom-Mow and Virizion are better choices.
