Button delay affects some people more than others. I personally suck at adjusting to button delay for example, if it's the same for me and my opponent, my opponent can be better at adjusting to it. Therefore some people are better online players than others, but that has nothing to do with SKILL because real skill is tested in tournaments, where they are is no lag and that skill has no meaning.
That's in addition to some characters being more affected by button delay than others. The more you adjust to online play the more you are off your timing in real life. If I know the timing of PK thunder sweetspot at all the right angles offline, and I'm such a master at it that it's reflex and I don't think about it, by using those reflexes online I would only suicide. If someone who sucks at PK thunder and THINKS about it before they use it, they won't use any reflexes and tend to suicide less. An example of why sometimes being better offline can sometimes hurt you online.
Online play punishes you for having fast reflexes and knowing the timing of your moves inside and out.