According to KRD, Mortar and Cluster Bomb are OP in Intermediate at 3 stars. The reason why T17 would have them at 2 stars is simply to have the same weapon powers.
In Intermediate, you have 3 Clusters to begin with, and that is in fact the
only weapon in it that can predictably and easily do >90hp damage to a single worm in a common circumstance (a worm standing on a steep slope). So I can see the logic in limiting its power in a scheme closely based on [ Intermediate ].
But in Team17, you have to collect Clusters in a crate, and there are plenty of other weapons you can collect in crates that do about the same damage (Minigun) or much more. And experienced players will know to avoid making themselves vulnerable to Cluster Bomb when possible, anyway.
As for Mortar, I don't see the logic of nerfing it in either Intermediate or Team17. Even in the best of circumstances, which come very rarely, it virtually never does more than ~45hp damage in circumstances where you're not also damaging your own worm, or more than ~58hp damage (not including grave damage) when you are. And while it is theoretically possible to do 70hp or more damage to an enemy worm with Mortar, that is so exceedingly rare to happen in a real game that if you can pull it off, it should be celebrated, not nerfed. But if there is some way that Normal players discovered to predictably do a large amount of damage with standard Mortar, I'd love to know.
(Edit: On level terrain such as a bridge, the floating weapon glitch can be used to get Mortar damage of about 63.5 ± 10.8 hp (n=32). But that'd require using your only rope, and with worms starting at 150hp, that's not much damage to waste a rope on.)