I don't even know about the Thumper. Does it really shred hull compared to a Heavy Mortar?
It does when Thumper hits because shots are very fast (or very hard to see and try to dodge if the ship I am piloting is the target), while Mortar and Arbalest miss because they are slow and mildly inaccurate. I have seen Thumper kill weakened ships multiple times early in the game. Pirates has finished off my ships (flagship and AI alike) after they were severely injured. I have used Thumper few times when I was frustrated with slow Arbalest and Mortar combo (especially before I get Gunnery Implants and Fleet Logistics 3), or when I needed to stretch my limited guns. (I do not want to throw all of the kinetics on one or two ships and the rest have nothing.)
Early-game fights with pirates are unskilled or low-skilled, and the targets are flimsy. Thumper is viable during this part of the game.
I do not disagree that Thumper is sub-par and it eventually becomes obsolete. Mining Blaster is much the same way (since Hyperion is no longer an endgame asset).
Over all i think that Frag Damage for non-PD roles really suffers and should probably be taken out of the game
I make an exception with Locusts. The damage is so overwhelming that it is often an unavoidable auto-kill against frigates, and even capitals lose a big enough chunk of hull if all of the missiles hit there.