Generally I view a d8 hit die as the average option since d10 hit die is the tanky option for martial characters. But if a character lacks other suitability tools they still might be considered squishy. Primarily here I'd be talking about Bards and Warlocks that wear light armour and don't take any spells for personal defense. These classes can make choices to have more defense like going valor bard for medium armour + shield, or the mage armour invocation, but these choices are mutually exclusive with other options.
Other d8 classes like Cleric, Druid, Monk and Rogue aren't considered squishy due to having other defensive boons like medium or heavy armour, having Dex as a primary ability score to make light armour not suck, or other defensive features like wild shape for an extra pool of hp, uncanny dodge, dodge as a bonus action. I wouldn't consider these classes to be particularly tanky unless they really focus defense, but rather these make up the average baseline in my mind.