Theres nothing inherantly wrong with premades, in fact, they are playing the game the way it was meant to be played, and have been greatly failed by GPG for lack of a team pantheon.
However, there is something very wrong with premades who go out of their way to disguise it. Anyone who does that is just inflating their record and ego by stomping pug teams in meaningless games that nontheless tick their record up +1. This can be as simple as just lying when asked, but can reach even deeper levels of scheming. For example, my friend and I recently joined a certain 2v2 game (after hosting a lobby for several minutes we got bored and went looking for a game). The host left the 3rd slots open on each team. My friend and I took spots 1 and 2 on darkness, and the lobby was 4/6. But instead of starting the game, a 5th person joined, and the host kicked the person that had already been on his team. For about 5 seconds my friend and I were confused but by the time the game loaded we knew exactly what they had done. Turns out they had also modified almost all the game settings to suit their playstyle as well. This was just about the worst level of premade deceit that we had ever witnessed, and my friend cursed them out all game as we dominated them for half an hour (then my router skipped, I d/ced, and my friend still almost won because we were so far ahead at that point). But I digress. Deliberately trying to cover up your premade-ness is bad.
The fuzzy area, though, is instead of lying or decieving, simply declining to mention. My friends and I tell people who ask, but we don't mention it unless they do. This is because it takes sooo much longer to get games if you publicly declare your premadeness. Pubs don't want to play you because they know they'll get stomped. Even the scumbag-level premade teams avoid you because they reason: enemy premade --> higher chance of loss --> not worth risk --> back to pubstomping. Anyways, my team's comprimise has been to host "No Noobs" or "Pros" games to try to attract better competition, but declining to mention our premadeness unless directly asked.