Could Norway have afforded more capable destroyers?

Was interwar Norway simply too poor to invest in better destroyers?

No. Norway had limited resources, but somewhat more capable ships could have been funded without a massive budget increase.

National poverty is not treated as a complete explanation for the Sleipner-class compromise. Norway had enough revenue that a better-protected and more effective design was financially conceivable. The claim is limited to a modest improvement rather than a fleet able to match a great power. That distinction turns the class into a procurement-priority problem rather than an unavoidable consequence of economic weakness. The relevant measure remains how well the ships support their intended mission.