Believe or not, design "trends" will come and go. Probably forever. No amount of bellyaching is going to change that. What you did in March might just go out in May. That next big thing you're working on that only you do might just make headlines in August and soon everyone could be taking a piece of that pie.
We're taught to buck trends and think for ourselves. To laugh in the face of groupthink and carve our own way. But this kind of thinking can also lead us to the immediate dismissal of those things worth keeping (or at least worth thinking about).
Bemoaning common practices can blind you from gleaning any actual "good" from the "bad".
Some were too busy throwing stones at circle avatars to actually try and understand what made them work. The fact that the human face is more closely round than it is square and that by placing it in the center of a circle, your focus is more directed towards the face seems to be a moot argument in the war on "trendiness". Furthermore, it also more effectively obfuscates the background by masking it over a shape that clips hard lines and more intentionally places visual weight towards the center--the face. On images that might be less than 100 pixels, this can make all the difference.
Not only that, using an ellipse can more quickly delineate content vs. user account. In the feed to the left, you can understand at a glance that this specific visual language classifies users by ellipses and content (let's say photography) by squares. It also allows the blocks of text (comments) to breathe better by adding whitespace to the user's avatar.
Systematically stripping out gradients, textures, and light sources from user interfaces was a reaction to over-stylized, clunky, and altogether ineffective information architecture that relied more on gimmick than user experience. Its refocus on layout and typography has been refreshing in a number of cases.
But this isn't just about whether or not you should use circle avatars or "flatten" your UI. Just because the hierarchy works in this medium doesn't mean it applies to all products. Removing all the dimensional character from your work could leave it feeling lifeless and ineffective. Add that to the fact that you don't want every site to look exactly the same. But more importantly, I'm not here to argue for their use. What I would suggest is that you take the time to ask why some motifs are making the waves that they are; getting to the root of why something might look cleaner, more intuitive, or just plain better than solutions from six months ago within the community. Sometimes the "Why" won't hold much weight. In that case, I might suggest discarding the practice altogether unless it's stylistically required.
Your discernment in seeing the difference between what's trendy for vanity's sake and what's actually a better, more well-conceived solution is paramount to your growth as a designer.
Falling prey to groupthink isn't bad when the group is on to a better answer.
Sometimes we've done things wrong. Occasionally we'll even admit it.