Can we take a minute to appreciate how kino Mt Mortar was?
It's a completely optionnal dungeon - in fact it takes less than 7mins to cross in your regular playthrough or you can completely skip it with surfing - and yet it's one of the most ruthless places in the game.
You need :
Strenght, flash, surf and waterfall in order to explore it fully
Plenty of repels against wild pokemon
Likely many tries because it's so deep

And the reward at the end? Not only you get a bit of lore for the pokemon world (what happened to that old Karate master, it's even hinted in Kanto in case you skipped Mt Mortar) but also a super rare Jotho mon, Tyrogue.. That alone makes it super worthwhile.

The only downside is that you need to leave room for one pokemon in your party. That's just frustrating and bad level design.