It is not very weak, no. The "physical strike" (Shock, Free Energy, etc.) formula is exactly the same (it also takes the defence into account) provided that it has a power of 15 (Free Energy is in this case for instance).
Damage Free Energy = (1.5 * Atk - Def) * Bonus
Damage Jump = (1.5 * Atk - Def) * Bonus
Damage Shock = (3 * Atk - Def) * Bonus
The difference is that Jump costs no MP, is available from the start and has the feature of making Freya untargetable for some time.
High Jump raises Jump's damage to "(2 * Atk - Def) * Bonus", which is the power of Scoop Art.
Of course, if you take damage boosts into account, then the attack command can be tremendously better (there's a reason why it's used in speedruns), but it requires a better preparation and being in front row.
However, the trance damage formula is really bad indeed. The added randomness (similar to Quina's attack) makes it very weak and it's not even counter-balanced by the multi-targeting since the damage are spread (so "more enemies = less damage per enemy").
In my mod, Spear has the same damage formula but it takes the properties of the weapon into account (elemental damage, killer abilities...) so it can be useful even in late game for damage. I had to edit the game's source code for that though. I also fixed the trance damage, removing the added randomness.
If you want to modify Jump's damage formula, you may need to edit the game's source code (that's the CIL code section, but I advise you to use an external tool, dnSpy, instead: it allows to modify the source code as well but it's way more advanced) but that depends on what you want:
1) If you want to make jump's damage use a different already existing formula (like Shock, where you control the multiplier), you can simply change it in the panel "Party -> Spells" and look for the abilities "Spear" at the end of the list: the first one holds the normal damage formula, the second one holds the trance damage formula.
2) If you want to use a new damage formula or if you want to modify Jump's "Freya leaves temporarily the field" principle, then you need to change the source code indeed. Specifically, you need the edit the class "btl_calc" (in dnSpy, open the game's "Assembly-CSharp.dll" and search for "btl_calc" inside the folder "-").
Here is what it looks like (I've rewritten the class in order to add comments and readability, but it's still C#). You can see that there is a case 48 and 83 for the formulae of Spear and Trance Spear in the main method CalcMain.