How far was the escape pod from the cockpit? I can't recall now.
Humans can be exposed to a vacuum for brief periods of time and come out the other end fine, but the key word is brief.
Footage of NASA test subject being exposed to a vacuum
Not very far, only a few feet. But I'd still wager that Joker would have needed immediate medical attention even after a brief exposure. Wikipedia has some pretty grim things to say about space exposure.
"Ebullism, the formation of bubbles in body fluids due to reduced ambient pressure,[2] is the most severe component of the experience. Technically, ebullism is considered to begin at an elevation of around 19 kilometres (12 mi) or pressures less than 6.3 kPa (47 mm Hg),[2] known as the Armstrong Limit.[1] Experiments with other animals have revealed an array of symptoms that could also apply to humans. The least severe of these is the freezing of bodily secretions due to evaporative cooling. But severe symptoms such as loss of oxygen in tissue (anoxia), followed by circulatory failure and flaccid paralysis in about 30 seconds.[1] The lungs also collapse (atelectasis) in this process, but will continue to release water vapour leading to cooling and ice formation in the respiratory tract.[1]
A rough estimate is that a human will have about 90 seconds to be recompressed, after which death may be unavoidable.[2][3] Unconsciousness is likely to occur within 14 seconds, primarily due to the much lower pressure outside the body causing rapid de-oxygenation of the blood (hypoxia).[4] In 1966 NASA volunteer test subject Jim LeBlanc lost consciousness after approximately 15 seconds of being accidentally depressurised in a ground-based depressurization chamber.[5][full citation needed] If a person is exposed to low pressures more slowly, hypoxia causes gradual loss of cognitive functions starting at about 3 kilometres (10,000 ft) altitude equivalent. Less severe effects include the formation of nitrogen gas bubbles and consequent interference with organ function (decompression sickness), which is less severe in space than in diving. Meanwhile, reduction of blood carbon dioxide levels (hypocapnia) can alter the blood pH and indirectly contribute to nervous system malfunctions. If the person tries to hold their breath during decompression, the lungs may rupture internally."
Also, the Collector Death Ray that was carving up the bridge probably was pumping out some pretty nasty radiation. And I don't think that t-shirt was lead lined.