Well, he is stupid, Momo is a grown up now with an "ancient" devil fruit that breaths fire.
Yeah, Oda is trying to tell us three things here:
1. Grown Momo is actually strong as fuck and growing stronger
2. Momo's fruit isn't a failure, and is at least somewhat near Kaido's in power
3. Green Bull is weak to fire
I thought Green Bull might have more fire resistance, but it looks like this is why Sabo cleaned up so easily in Mary Geoise: Green Bull just can't take the heat. Even though it looks bad for Aramaki, these are just bad match ups for him. Unfortunately... sooooo many people in One Piece fight with fire.