No big mystery here. It means that there is no shine or brilliance or that something is uninspiring. Often used in reference to performances and productions.
Yesterday’s match saw a lacklustre effort from the league leaders who spent most of the game on the defence and only had one chance at the opposition goal.
During the election debate the Vice President was distinctly lacklustre, giving brief and unconvincing replies and failing to inspire his supporters.
It can also be used to describe hair and I guess, by extension, aspects of grooming.
It is not a word I use or have heard used much in spoken English and tends to appear more in reviews or reports but what I like about it is that it sums up a particular state perfectly.