How much they matter depends on many circumstances. If you want to play in tournaments, then you risk running into matchups that are horrible for your character. If you only intend to play in locals, then tier lists matter a bit less, since people at the local might not play those characters that counter your character. If you intend to go to larger tournaments, then chances are you will run into some terrible matchups sooner or later and your character will hold you back to some extent.
There are many, many examples of low tiers and mid tiers doing really well at smaller tournament scenes, so it certainly can be done. It's up to you to decide whether it's worth it or not. You could main Kirby (or Game & Watch, who is a low-mid/mid tier) and have a secondary to deal with bad MUs, perhaps?