My general rules for evaluating by-elections:
1.  It's always better to win than to lose.
2.  #1 aside, unless the additional vote is valuable in the current parliament, they seldom have a practical impact.
3.  The predictive value of by-elections, which tend to be low-turnout, is highly variable.  Governments have lost by-elections and gone on to win re-election, and vice versa.
All that said, Lac-Saint-Jean is one of the more useful by-elections in recent memory, as a proving ground for the Liberals' belief that they can expand their Quebec reach next election even if they lose a bit of ground elsewhere.  Clearly a positive sign for them.
In terms of what this says for the other parties, ehh.  People convinced that Singh will tank the NDP's numbers in Quebec won't find much consolation here, but the NDP's fortunes were already waning before he won, so I'm dubious this means much on that score. The Tories lost a seat, but I don't think Scheer ever banked on big Quebec numbers.