Is it important to main a top-tier character?

[High Entia]

Top tiers are important, but not using Sheik as your best and only character isn't the be all end all of your competitive life. You should main characters you feel fit your playstyle the most. I dual main Lucas and Shulk because I love playing both of them, they fit my defensive and zoning-based playstyle, and, at least based on my personal matchup charts, they cover each other's poor matchups fairly well.

However, I'd recommend picking up a top tier secondary as a fallback or to cover bad matchups. For example, I secondary Rosalina and Ness. Top tier secondaries can really get you out of a pinch if you're really struggling with a matchup or you're just not feeling it with your non-top tier main that day.