Is competitor monitoring worth it?

I'd say yes but don't get hung up on it. It's good to be aware of what's happening in the landscape but it's easy to get sidetracked or diverted by what others are doing and that's rarely a good thing early on. Stick to testing and iterating.

Nope. A startup with growth potential is serving less than 5% of the total addressable market for their product – often more like 0.1%. Its challenge is reaching product-market fit and then cost-effective distribution/awareness.

When prospective customers provide competitive feedback, consider it. Beyond that, don't sweat competition.

(If you get over, say, 5%, that changes: it needs to either grow the TAM or take market share from other vendors. That's not what most people would call a startup, though.)