Honest question about skill deciding matchups.

I see people say all the time that some champions are decided only by their mistakes instead of the other player doing well/better.

But what draws the line of difference?

If a champion aims a skillshot correctly (AKA, not missing the skillshot altogether in the first place), but then said enemy champion manages to dodge it, was the first player bad for missing it because they did not account for potential dodging and adjust their aim for that, or create a situation where said enemy champion would NOT have been able to dodge it? Or is the second player good for dodging said skillshot which would have hit them if they did NOT dodge?

If a champion dives in 1v2/3/4/5 and manages to get a favorable trade (either not dying and killing at least one, or dying and killing at least two) and all players are equally decent at the game, does that make the first player better for being able to outplay and dodge everything said enemy team did successfully? Or does that make the other group of players bad for not being able to properly shut the first player down even when they were as a group because they didn't properly calculate the skillshot travel time versus the way the first player would try and dodge or avoid said skillshot?

So on and so forth.

What do you guys say?

I am talking about people of EQUAL skill level, not a matchup where one side is clearly better than the other.