There is no credible evidence in the available sources that the alleged Charlie Kirk shooter wore a Trump costume in 2017.
The alleged Charlie Kirk shooter wore a Trump costume back in 2017.

Utah officials share video of Charlie Kirk shooting suspect as governor pledges death penalty - as it happened - Charlie Kirk shooting | The Guardian
Source: theguardian.com
Key Evidence
The Guardian, PBS News, The New York Times, Sky News, Newsweek, The Independent, and AP News all report extensively on the shooting suspect and investigation but do not mention any Trump costume from 2017 or otherwise. The suspect is identified as Tyler Robinson, but no background details about costumes or related claims appear in these reports.
What the Evidence Shows
I looked through multiple recent and authoritative news reports about the Charlie Kirk shooting and the suspect involved. These sources focus on the manhunt, the suspect's identity (Tyler Robinson), and official statements from authorities and political figures like Donald Trump. None of them mention or provide any information about the suspect wearing a Trump costume back in 2017 or at any other time.
The coverage is centered on the shooting event itself, the investigation, and political reactions, without any reference to such a costume claim. Given the high-profile nature of this case and extensive media attention, if such a detail were true or confirmed, it would likely have been reported by these major outlets.
Therefore, based on current evidence, this claim remains unverified and unsupported by reliable sources.