Why is Glastrier not good in the tier? What Pokemon outclass it in the role(s) it wants to do?
It's not a thing of being outclassed, more like "there's no place for Ice types in the tier outside of Snow. And even then it's a questionable choice".
There's not really a pokemon that fills Glastier's role atm as a bulky swords dance breaker, but by virtue of being an Ice type it can't really do said breaking reliably against balance. You don't really do progress against defensive teams either, since most of them use either diancie (who can outbulk you and outdamage you with Diamond Storm's defense raises) or Quagsire (who can take a hit, Toxic you and recover stall). You could say that the Ice disadvantage is patched by Tera, but that makes it dead weight for most of the match. Weather being prevalent doesn't help matters either, since you're vulnerable to the 3 setters, only really taking advantage of snow (which again, is quite niche).
There's probably a team that can make it work (maybe with wish support and LOTS of hazard removal), but you're better off just using other physical attackers in the tier. If you really want to use an Ice type, use cloyster