Letter ё is optional, it is usually replaced by е. We have a number of questions about this letter on this site, for example: Using written е instead of ë,
Is 'ё' a distinct letter, or is it just 'е' with a diaeresis?, What are all the times where "е" changes to "ё"?. In short, letter ё is always used only in dictionaries, text for beginners and children, uncommon foreign words (like geographical names) and in cases when difference between word with ё and е is not obvious from the context (like небо (sky) and нёбо (roof of mouth)).
For modern reader it is definitely лёгкий. As for Pushkin times - I'm not sure what was the pronunciation at those times. Usually you notice the difference when two words stand at the end of the verse and should be rhymed (like Ольга/фольга pair which currently have different stresses, but at the beginning of the XX century both words had a stress on a first syllable).
But even when two words happen at the end of a poem's line, it is hard to tell should those words be pronounced with ё or with е. A long article about letter ё has the following example (by academic Vinogradov) of Pushkin's Poltava:
Тесним мы шведов рать за ратью;
Темнеет слава их знамен,
И бога брани благодатью
Наш каждый шаг запечатлен?
According to Vinogradov here both words are likely using "е" (знамен/запечатлен), but we cannot tell for sure.
Another example where there is е and not ё is verses from Krylov's fable "Лебедь, Щука и Рак" (example from Wikipedia article):
Когда в товарищах согласья нет —
На лад их дело не пойдет
Here, in Krylov's times пойдет was pronounced as е as can be seen from the rhymed нет/пойдет. Interestingly, in modern textbook for pupils пойдет in this verse is written using ё: пойдёт.
Looks like it was pronounced as легкий in Pushkin times. According to Wikipedia article the letter ё's was printed for a first time in 1795. Before that time a number of letter combinations served to resemble this sound (о, ьо, їô, ió, ио). However Google Books has a book printed in 1792, which is entitled "Новый легкий и самый безопасый способ лѣчения чесотки". If it has to be pronounced as ё there should be one of this combinations used: о, ьо, їô, ió, ио. Since it is not so, it has to be read as "е".