This is certainly a problem, but I find it happening less on "special interest" boards. I don't think it's too far from Reddit in this respect. On /r/funny you might expect something like this to happen, with the top 2 comment threads being puns, or unenlightened comments. But on /r/EngineeringStudents or /r/compsci or /r/lisp ... (subs I vaguely remember) that's less likely to happen.

Right now on /leftypol/ there aren't many threads of what you describe. They do exist, but they aren't numerous. What you describe is more similar to 4chan's /b/, but even 4chan's /g/ has "tightened up" and is being steered more toward discussion (still with racial slurs but not entirely focussed on them). A notable exception is the "DESIGNATED SHITTING STREETS"/ "POO IN LOO" meme, though.

I don't know if these are just my experiences.