Yes, you can stack notables from cluster jewels.

Do cluster jewels stack keystones?

Keystones do not stack. Only cluster notables stack.

Are cluster jewels good?

Small Cluster Jewels were introduced in PoE's 3.10 Delirium League and have since become build-enabling for a lot of builds. Whenever they are not mandatory, they almost always are extremely nice upgrades to have and can function very well even when you aren't insanely rich and can't afford the best combinations.