It Depends on the state and court
How much mental capacity is required depends on the jurisdiction. In every state, there's a law that describes who has to serve jury duty, and who is exempt, and if a style of dress is prescribed. In the following, I will grab a couple of random states to illustrate, using https://www.juryduty101.com/ as the main source, because they give a good overview for all 50 states but are not fully reliable for all details. Look up your proper state statute using the reference links from the site.
Do also note that courthouses might have additional requirements that might be higher or more specific, especially in the dress code (some might require to wear face masks, others don't) or current health status (like testing negative for COVID). It's required to read the call for jury duty for those. As a random example: the Eastern District of New York tells its expectations (almost mimicked by most other courts):
Proper court attire is mandatory. No jeans, shorts, tank tops, sweats or other very casual attire is permitted. Dress "business casual" or "business" for your appearance.
In connecticut, there's a rather simple list of who's exempt from Jury Duty in the first place. The page is listing, among others, disability as an excuse from Jury Duty, and you need to qualify in the first place. The Qualifications demanded from a juror, among others, are this:
- have the ability to read, speak, and understand English
- be of sound mind to the capability of "rendering satisfactory juror service, being able to perform a sedentary job requiring close attention for three consecutive business days for six hours per day, with short breaks in the morning and afternoon sessions."
The bolded portion is why people with ADHD might get an excuse in Connecticut, while the sound of mind part allows someone with a mental disability to claim disqualification.
In arkansas, the qualifications demand the prospective juror to consider themselves of sound mind and good moral character - which appears as a self-estimation. The actual legislation for disqualification for sound mind and good moral can be found here, and doesn't make it a self-estimation but makes it appear a thing of fact, without elaborating how that is determined.
missouri, boldly demands a juror must not be mentally or physically ill or infirmed. or... actually that is too broad of a characterization, as it omits some crucial words:
MO Rev Stat § 494.425 Persons ineligible for jury service.
494.425. The following persons shall be disqualified from serving as a petit or grand juror: [...]
(8) Any person who, in the judgment of the court, is incapable of performing the duties of a juror because of mental or physical illness or infirmity. The juror or the juror's personal representative, may provide the court with documentation from a physician licensed to practice medicine verifying that a mental or physical condition renders the person unfit for jury service for a period of up to twenty-four months.