some pleco species are more aggressive than others, what type do you have? there are hundreds of them.....its not too common for a basic common pleco to be aggressive.....
i have had an aggressive pleco that i was forced to sell.....too bad cause he was a beautiful gold spot pleco.....he was attacking my tinfoil barbs to the point where he was ripping off their scales, and the tinfoil barbs were more than DOUBLE his size!!
some plecos get aggressive when they get to breeding size, but the aggression is rarely seen before then.....when they mature to breeding size they kind of have a change in personality.....not really anything you can do about it unfortunately
i have even seen plecos literally latch onto the sides of other fish and suck the slime off them....
so to answer your question....it is not uncommon for a pleco to be aggressive