Read the record. There was also a recent movie on the subject. Any court that denies a defendants rights to testify in their own defense is a kangaroo court.
This is the 167th footnote in the Wiki entry for Mary Suratt.
Neither Mary Surratt nor any of the other defendants testified on their own behalf. Although some sources claim that they were prevented from doing so, this is incorrect. At the time, the federal government and 35 of the 36 states did not permit defendants in felony trials to testify on their own behalf. See the discussion in Boritt and Forness, p. 352-353.
If accurate, this would mean she wasn't allowed to testify, but not because this was a kangaroo court. She would not have been allowed yo so testify in almost any court at the time.
The rule appears to have been a hangover from common law, in which nobody was allowed to testify who had an interest in the outcome. Since most if not all felonies were originally capital crimes, the accused obviously had a pretty strong interest in the outcome.
The qualification of criminal defendants to give sworn evidence if they wished came last. The first statute was apparently that enacted by Maine in 1859 making defendants competent witnesses in prosecutions for a few crimes. Maine Acts 1859, c. 104. This was followed in Maine in 1864 by the enactment of a general competency statute for criminal defendants, the first such statute in the English-speaking world. The reform was largely the work of John Appleton of the Supreme Court of Maine, an American disciple of Bentham. Within 20 years most of the States now comprising the Union had followed Maine's lead. A federal statute to the same effect was adopted in 1878, 20 Stat. 30, 18 U.S.C. 3481. Before the end of the century every State except Georgia had abolished the disqualification. 6
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=365&invol=570
Her trial may indeed have been, probably was, a kangaroo court. But her not being allowed to testify did not prove it so unless every other criminal court in the nation outside Maine also qualified as such.
Nor is her having possibly had an unfair trial any evidence for her innocence.