1) don't critisize, create instead 2) Criticism is good, even if is kind of mean.
But I think this a false dichotomy. Words and tone do matter. If you put the object of your criticism in a defensive position, you are unlikely to effect any change at all. This is not some new insight; it's been well understood at the very least since Dale Carnegie. You might acheive the goal of making yourself feel good, but then that certainly isn't worthy of any external respect.
What's the difference between: 1) You app is ugly and it fucking sucks. If you used abc to do xyz, at least I wouldn't be vomitting.
and
2)Nice effort on your app. You might consider using abc to do xyz. It might improve the aesthetics and usability some.
Some might say it says effectively the same thing, but the there is a world of difference in the way the reader reacts to those statements.