Tinted glass does offer better thermal performance, especially in a climate like Montreal's. It reduces solar transmission in summer and helps retain heat in winter, which lowers air conditioning and heating costs. But honestly, it's also an economic thing: it's cheaper to mass-produce than clear glass with specialized treatments, and developers love it because it gives a "modern" and "premium" look without huge investment.
Fashion plays a role too, you're right. Montreal follows trends you see in other major cities, and smoke gray / tinted glass has become the signature of contemporary construction pretty much everywhere. It does create a visual uniformity that can feel pretty dull. I get your unease with the skyline - it's true that it lacks variety and character compared to older buildings with their brick or stone facades.
If you find it depressing, know that Montreal architects and urban planners talk about it too. Some more recent projects try to incorporate other materials or play with shapes to break it up, but yeah, for now gray glass is still dominant for the practical and commercial reasons I mentioned.