Not necessarily. Basic flat insoles are simple foam footbeds with minimal arch support, and they're standard on most entry-level cycling shoes. If you're a casual rider sticking to shorter rides, they'll likely feel just fine.
The real advantage of a basic insole is that it's easy and affordable to replace. Many riders who already use custom orthotics or aftermarket footbeds (like Superfeet or Specialized Body Geometry) actually prefer starting with a flat insole so they can swap in exactly what works for their feet.
However, if you're planning longer rides or performance cycling, the lack of arch support can lead to foot fatigue, hot spots, or numbness. In that case, you'll want to budget for an aftermarket insole upgrade or look for shoes that come with contoured or interchangeable arch inserts right out of the box.