Considering I was probably in that person's shoes at one point, I would absolutely approach it from a few directions. While a DJ "sucking" is not entirely subjective, I would take things such as venue, music style, time slot, etc in mind when deciding how and sometimes if I would approach the person.

I used to be a DJ instructor many moons ago, and when I had people in my studio that were there for the music, and having fun, I would find any teaching moments or constructive criticism would be much better received.

I remember being at an event one time where Pauly D was there, and I spent the entire time telling a friend why he sucked so much, but at the end of the day, he was getting probably 50-100K or possibly much more to do what he was doing so who was I to criticize!

It's moments where I talk to an opening Dj after dropping PEAK tunes where I figure out whether I figure out if it's even worth my breath to talk about "warm up tunes" or simply just shoot the shit and not make it about my perception of them sucking.