I wouldn't really say that because Enel's fruit's specialty is literally lightning while Big Mom can spam several elements AND still showed similar feats to Enel's main assets which definitely ain't bad.
Sure, I meant if we solely compare Big Mom's lightning to Enel's.
That's literally the point - he needs a tool to replicate this feat.
Big Mom achieved this feat on her own, with her homies. And it's not like that's all Big Mom can do. Perhaps she can even perform a similar attack like Enel's Raigou? If she can do that, it's a proof that she's able to eradicate whole islands from its mere existence...
Also, add her ability to add souls to every single object makes her feat even more impressive since she also has Prometheus and Napoleon on her side as well.
Enel doesn't need the Maxim to do that in general. He needs the Maxim to do it in sky island. As the citizens said, they have never seen dark clouds in Sky island. Enel needed the Maxim to create thunder clouds. On the other hands these thick clouds were at Big Mom's displosal when she arrived, then she made Zeus eat the clouds to regain strength and then used those clouds in conjunction with Zeus. If Enel was there, he would've made use of these clouds just as well.
Can she perform something like Raigou? Probably. If she fuses Zeus with the thunder clouds. But yes, since Enel is lightning itself, his capabilities in that specific domain should be greater. Just like Ace' Mera Mera no mi.
But yes, overall, Big Mom's ability is very great, as she, as you said, can use multiple elements on a larger scalre and even combine them, control the weather and cause storms.