It's hard to climb when you have a large pool where everything is pretty equally good, but none are outstanding. I may be sub 50% on my worst roles (support I'm like 0-3 this season) but I'm way higher than 50% on my main and probably like 60% on my second role. My main role is so strong just because I play it ALL the time. I know pretty much every matchup with the 2 main champs I play so I can do well in almost any situation. If I'm stuck on like my 3rd or 4th best main role champ then my win rate would probably drop, or if I'm stuck on my worst roles. There's nothing wrong with being decent at everything though, just that your climb will take a lot longer.

Figure that there's maybe 10% of games you lose no matter what, 10% you win no matter what. Out of the remaining 80% there's 20% where the game is in your favor but not extremely so, 20% where the game is somewhat against you, and 40% where things are pretty close and what you do will have a huge impact. When you're playing your main role on a main champ, due to familiarity you can probably win almost all of the +20% games, a good chunk of the 40% games, and some of the -20% games. If you're playing as a fill with decent skill all around then you're probably winning the +20% about as equally as you lose the -20% so that cancels, so all you're left with is trying to win the 40%. If you're only slightly above average then you'll climb because you should win those even games more than you lose, but it won't be fast.

You're probably a better OVERALL player than most people in your games, but you may be slightly weaker than whoever you're in lane against if that's their main lane, which makes things a lot harder to climb.

So it just depends on what you want out of ranked and how quickly you want to climb. If you enjoy filling roles and enjoy playing a lot of different champs then do it. Nothing is stopping you from enjoying yourself. If you want to climb and climb relatively fast though it'd be easier to pick a main IMO.