Come on man, you of all people should know that mountain level isn't all that impressive in the Grand scheme of things when Top Tiers should be at minimum Large Island to Small Country level.... So we really going to pretend that one tier down from Top Tier isn't at least capable of Mountain Level energy output?
Well it depends on what top tier we are referring?
Is it impressive in comparison to people like Mihawk, Whitebeard, Akainu, Garp, not really, they can dishout that level of power just by flexing their muscles.
Mihawk's attack was borderline Island level btw,
https://worstgen.alwaysdata.net/forum/threads/dracule-hawk-eye-mihawks-insane-feats.4370/
But it is definitely impressive for someone like big Mom whose serious signature, even desperate attacks have mountain level yields at the maxumum so far, at best she peaks at Small Island levels at least in my opinion anyways
I can try to give you rough estimate of power out of all top tier based on my understanding of manga so far
Prime Whitebeard -> Small Continent Level
Mihawk, Prime Garp and Gol D Roger (due to portrayal) --> Large Country level
Akainu Aokiji Kizaru Hybrid Kaido Fujitora --> Small Country level
Oden --> Large Island level
Big Mom Base Kaido Current Zoro --> Island level
Luffy --> Large mountain level