DMG Marking

[Cheesegear]

...if the explicit options for protecting others are weak enough that the best defensive option is doling out damage, then aren't those defensive abilities underpowered?

No. If your DM is playing fair, then most appropriate hostiles simply aren't that strong against a reasonable party. D&D just isn't that hard. It just isn't. It isn't that "Defense is underpowered." It's that;

"Defense doesn't mean anything beyond early Tier 2 because bounded accuracy and the entire game design, says it doesn't. Action economy says enemies don't have actions when they're dead. So that's the game-state to aim for."

For this not to be true, the DM has to make encounters where dropping the hostiles to 0 hit points isn't a win-state...And just isn't how D&D works. Especially because a player dropping to 0 isn't even neccessarily a lose-state.

Then, if your DM isn't playing fair, and sending you inappropriate hostiles, then no defensive measure you bring is gonna be worth it, because the DM is stacking the encounters against you, and most likely takes your abilities - whatever they are - into account when doing so.