Is the title, Tanaka-kun Is Always Listless, a good translation?
Not only it's a good translation, but it's also the correct translation.

Logan has already explained the construction of the word


  けだるげ is technically a combination of けだるい and 気 using this somewhat uncommon construction.


Using this as the base, searching for the definition of けだるい returns:


Jisho: languid; listless
Japanese Wiktionary: listless, lazy


The conclusion is, echoing Senshin's comment:


  The English title is a literal translation of the Japanese title; there's nothing to read into here.