I feel it is, yes (this is personal opinion, unrelated to rules discussion). Hitting a super high AC is acceptable in the 'there's always a gap' kind of sense... brain-powering your way of a mental crush from an overbearingly superior force, not so much. If anything, your barbarian is a prime example of why one of your heal-capable characters might consider carrying lesser restoration with them as one of their prepared spell, going into nasty combats, or just in general - expressly, to help the characters that cannot help themselves.
There's also the opposite case, where say a level 20 archer or wizard casting firebolt can miss a shot on a prone&unconscious commoner (or elephant or some other large+ monster with low AC) 10 feet away (so attacking at normal) if they roll a natural 1.
As you said this is all personal opinion, but idk our characters are typically the heroes of the story, yeah? Being in the exact right location (there's a rock or some rubble to hide behind) to avoid a fireball, or being so stubborn/finding inner strength to resist a mental crush seem like well within the realm of possibility to me. Also, again, I dislike the lack of progression of saves in 5e, so I like anything that can help that. And finally, rolling a natural 20 and succeeding regardless is fun, and doesn't really break anything for saves (unlike skill checks).