NO! Stupid Gamefreak! You make us Typhlosion, Feraligatr.. then this thing!!
I must admit though, Meganium does moderately well in rain, lets look!
Meganium (M) @ Leftovers
EVs: 252 HP / 128 SAtk / 128 SDef
Modest Nature (+SAtk, -Atk)
- Giga Drain
- Hidden Power [Water]
- Light Screen
Yep, the rain makes this better.
Giving Meganium fire resistance, means that he can function as a much better wall and tank with that weakness being gone, while having HP Water as a powerful coverage move, since he would have to use a regular 70 base HP or Ancient Power as a coverage move before, as Meganium lacks a strong special movepool. Counter + Light Screen is a neat trick, with Giga Drain to keep him going.
Meganium (M) @ Chesto Berry
EVs: 252 HP / 128 Atk / 64 Def / 64 SDef
Adamant Nature (+Atk, -SAtk)
- Bullet Seed
- Dragon Tail
Well, this gets no rain support, period.
Earthquake is coverage, Bullet Seed is STAB and sub breaking, Dragon Tail is for phazing, Rest is for healing off damage and getting woken right back up from your chest berry.
Overall, specially Meganium gets quite the boost, physically, he gets a fire resistance...
Rating for the rain - 6/10