Not someone that voted on your reply, but I think i can shed light on some of the reasoning people have. While you haven't indicated that you plan to do any of the things the OP was referring to, you have indicated a plan to do things commonly complained about relating to smurfs.
You have, in your own words, said you plan to do "crazy stuff" and will "get real weird with it" on these second accounts. This implies that you will be doing things that you know may be poor choices. Because these choices are all being made on a second account, you can rest secure knowing that your main account is safe in whatever rank it is. If things go bad, you just ignore the extra accounts and never play them again.
However, the other players in your matches are likely to be on their main and probably only accounts. These players now have the integrity (and fun) of their ranked experience affected by your desire to try "crazy stuff". If your "crazy stuff" is bad, you could say you are 'helping' 5 people on the enemy team and only 'hurting' 4 people on your team, and vice versa if it turns out good. I would not agree that any of these people are benefiting at all outside of a bit of LP on the record books. Having a feeder (intentional or otherwise) impacts the enjoyment of the match even for the winning team. There's a fine line here between meta-slaving and picking what you like, so I can't really draw a line and say "here be feeding".
Let's relate this to another game. Suppose a pro basketball player, let's say Lebron James (Is he still good? I don't follow basketball), decided that he didn't like having to play games on the pro schedule, or maybe he decided he wanted to try all 5 positions, but doesn't want to risk his pro status by having bad games while he's just trying crazy stuff. So, he puts on a wig and registers himself as Norbel Smith (it's a magic wig, nobody can tell it's him) in the local community league, which has divisions and tries to pair people up by experience and skill level and will move you up and down over time. Now he gets paired up with all the other players without experience, instead of the division where former college players start. Eventually, he should get promoted into these leagues because he's just so much better than everyone else. If he's trying crazy stuff, maybe he doesn't actually promote out because he's playing so bad his teams get way behind and he can't fix it by being more serious later in the game (not that he cares, he's just screwing around on the weekend. he knows he's a pro). Either way, he's ruined the games for all the inexperienced people who signed up for some moderately serious, evenly matched basketball games. The analogy might be a bit stretched, but I think it highlights the general sense of what a smurf really is and how ridiculous it would be in more traditional competitive sports and games.
If you only plan to play these extra accounts in unranked games, the problem is slightly diminished, but not gone. Everyone needs somewhere to practice things, and unranked games are considered the right place to do so. However, there's still some expectation of putting forth a level of effort, unless you have a full group of 5 that all agreed to just screw around. (It would be nice if there could be an unranked queue for "real" games and "fun" games, but that's not likely to be feasible from a queue health perspective.) You may put forth effort most of the time and be polite, but there will be a temptation to care less about your behavior in these games, since you can just go back to your main.
mains dominating my matches because no one knows how to play against them