This seems to be about taste in the end.
The question what a frontend developer should be will most likely be different with each person you ask.
Personally I would like them to understand it. But to me it's way more important that the person has a clear opinion on how things should look, behave and if possible understand what she/he is using and how it works.
The rest is teachable and will be done anyway in code-reviews / live coding sessions.
In general I am not the classic asshole throwing random shit at you in the interview. It most likely reflects in the level of payment. Even there I just want to here why I should pay a person more and we can actually define baselines for raises.
Anyhow ... my opinion is yes every dev should know data-structures it's important, but it's way more important that they learn from, communicate with and educate each other.