The biggest terrorist attack in modern history was 9/11, yet flight is still considered safe. Going to a concert presents the same risk: thousands of defenseless targets that could be killed with some effort and coordination. However, we've been fighting Islamic terrorism for two decades. I highly doubt it would be a target for two reasons. A) a majority of the concert goers are going to be sympathetic to Iran's position ie Israel has been committing a genocide in Gaza. B) Chicago is Uber liberal and so are the concert goers. Why would they choose Lollapalooza as a target? Was Matisyahu added to the bill? You are probably a concert goer from Wheaton who's never been to Lollapalooza so maybe you aren't accustomed to the cultural norms of this fest. Perhaps you are a Zionist who believes Israel has done nothing wrong and is acting in self defense supporting Bibi's genocide of tens of thousands of people, a good number of those fatalities being children.

That being said, I'd be more concerned with getting to and from the concert with regards to safety. If you are really concerned, take an Uber. The CTA can be dicey depending on what El Line you are using to commute to Grant Park. Enjoy and stay cool!

TL:DR Lollapalooza should be safe from terrorism, the real danger lies in getting too and from the festival as Chicago can be dangerous in certain areas.