For my money base Necrozma is the weakest legendary. It is literally crippled and all it does is absorb light which it can't even hold in without relying on another legendary's power. Any plant can absorb light better than Necrozma and Lanturn is canonically stronger than Ultra Necrozma at outputting light energy. Speaking of plants, according to the pokedex Shaymin "can dissolve toxins in the air to instantly transform ruined land into a lush field of flowers". It takes more power to create than destroy, so Shaymin is more powerful than all the legendaries that are just strong in battle.
EDIT:
The closest pokemon lore-wise to base Necrozma is Sunflora. Both sustain on light energy to live, however Necrozma is reliant on devouring light to even move (UM pokedex entry) while Sunflora only uses it as a nutrition source and can act freely in sunlight. Sunflora is more self-sustainable than Necrozma.
Also reposting a fact from last thread: Magcargo has a higher temperature than Ultra Necrozma (18,000 vs 10,000 degrees Fahrenheit), so Necrozma is really not that much relatively stronger than regular pokemon. Check the last thread out for detailed comparisons between Lanturn and Ultra Necrozma.