I think it's possible to argue this from two sides.
There's the optics of it, The Game of Politics: yeah, this isn't a good look. Depending on how the media approaches this it can be a big deal or not. Hillary needs to tackle this head on, do a few press events to show that she's fine and absolutely savage Trump in the first debate.
Then there's question of whether or not this is cause for real concern regarding Hillary's health, or the claim I've seen pop up a few times in this thread that this leads to real concern that Hillary might not be able to finish her first term: No. That's freaking ridiculous, unfounded, and should be called out for the trolling it is. There are dozens of reasons a person in fine health could have a dizzying spell that aren't Impending Death Syndrome. For some reason, people seem to be forgetting this in an election year.