That’s a little vague from a legal perspective.
For this scenario, the ‘waiver’ you suggest carries no legal weight as most local, county, and state jurisdictions defer air law over to the FAA and federal regulations under Title 14 of the CFR.
To begin with, an ultralight pilot acting as PIC of said aircraft is, by definition, directly responsible for, and the final authority on, the operation of that aircraft.
In order to fly said aircraft over an open venue of persons, you would need an FAA issued waiver of §103.15 plus additional waivers for other regulations, depending on the kinds of display in question eg aerobatics, etc.
Now you may still be held liable for property damage, injury or death with an accident if the root cause is determined to be pilot recklessness, incapacity or neglect in violation of §103.9. Circumstances also would dictate weather a local or federal prosecutor would seek to pursue criminal charges against you. A determined §103.9 or §91.13 violation would make you vulnerable to a civil tort, particularly wrongful death if your actions resulted in loss of life.
Companies promoting airshows may require performers to sign legal documents associated with her performances where the performer agrees that the promoter assumes no liability by the performer should an accident occur. I’m not totally sure on the details of that, but that’s not uncommon.