I think that level ratings do tend to be biased, but not necessarily due to just clanmates or friends. First of all, not enough reviews are written. Most levels probably never get more than one or two reviews, and some get none at all. This means any individual review has a huge influence. Everyone has somewhat different ideas about ratings, so even two people who like a level equally well can give it significantly different ratings. For example, I've heard many people say that 10's should never be given because 10 implies perfection and no upload can be perfect. I personally disagree with this, and in my opinion 10 simply means it's a top tier level that should definitely be downloaded. Therefore, I could love a level, and give it a ten, while someone else who also thinks it's an awesome level could rate it less.

Also, individual taste and experience can cause people's ratings to be affected by other things than the level itself. For example, I've played a game before on a new level, where I was frequently low on health, and always seemed to be able to get to the carrots just in time, which made the game very exciting. Because I had so much fun on the level, I gave it a good rating. However, if I had played the same level with a bunch of players who either weren't very good, and never were able to hit me, or who were insanely good and killed me all the time, I would have probably enjoyed it less, and therefore rated it more poorly. Ideally, a rating will give an idea of the average experience of everyone who played on the level, although due to the fact that so few people review certain levels, my individual experience may be the only one reflected, and people with the opposite experience may never write a review to balance it out.