What level of play would you define as high level?
Would high level be attending weeklies? Would it be playing in Elite online? Would it be trying to beat that friend that's better than you are? Or would you define high level as only the largest tourneys where all the best show up and compete.
The definition of high level certainly matters here.
I'd rank the above-mentioned thresholds as, at least for my case,
Trying to beat that friend that's better than you are is < Elite Smash
Elite Smash < attending weeklies
attending weeklies < competing in large tourneys.
As a lower tier main myself, I started to feel the tier when I went to weeklies. The players at the weekly were quite good, and most of them played higher tier characters and I did feel that I was playing from a disadvantaged state, given how hard I would get comboed or edge-guarded, and just generally being aware of the limited options I had at any time, as people there would punish instantly for any mistake, weakness, blunder, or hole they could find in your play or defenses.
From my personal opinion, tier lists start to matter once you break out of playing with just a friend group and attending real-life, in-person events where smash players compete against one-another in a tournament setting, such as a weekly. For anything less than this level of competition, it seems to not really matter, as player skill deficiencies, mistakes, and habits are the primary things I use to win when I'm playing in a more casual setting. Higher level play, it starts to matter a lot more what options your character is capable of taking at any particular level, as the other things you can use to your advantage start to disappear.
Players at that level simply aren't whiffing side smashes in neutral as often, opening themselves for a grab, for example. That's one less opening I have to work with.
That does not mean you cannot compete with a lower tier character at weeklies and beyond. It just means, in my opinion, that's where you start to feel the low-tier'ness, and have to work a bit harder to be on equal grounds with an opponent who chooses a higher tier character with a favorable matchup against you.