In pro wrestling, a clean win happens when a wrestler wins the match without violating the rules of the match or essentially wins the match through his/her own merits. As for matches that don't end in a clean win, it sometimes depends on what type of interference, how blatant it is and how it affects the overall impact of the match. For instance, using a foreign object in a match that's been pretty even and has been entertaining can be seen as acceptable by most fans, as is pulling the tights or using the ropes for additional leverage. I think that most of the complaints in regards to matches without a clean win come about due to outside interference.
In matches in which there are no disqualification, weapons and various other underhanded tactics are "legal" within the boundaries of that particular match. Outside interference is also legal but, again, that seems to be where most criticisms come from, especially if the match is a good one and both guys are working their asses off. I understand that because I get a little steamed myself sometimes if I'm watching a really compelling match and there are people who interfere. Minor interference such as reaching under the rope to trip a guy or hopping up onto the apron occasionally is pretty well tolerated. However, it's when you have someone actually physically getting into the match that can sometimes detract from it, especially if it's something that's been happening a lot in matches of some particular wrestlers. For instance, Cena vs. Rollins on Raw in a lumberjack match was kinda fun, but all the various interference detracted from the match for me because it's something we've seen too much of in Rollins' matches. We know he's capable and he's got the goods, we don't mind him using underhanded tactics, but interference should definitely be used in moderation and I think it's something that's used much too often in his matches.