Almost surely worth it

First, let me introduce the simple math on the DPR change:

I will be assuming the party of 3 characters attacking it have +5 on their main attribute (i.e., the one they use to attack), as is usual, unless they have a way to further increase their DPR through feats (e.g. GWM or Sharpshooter). I assume all of them are using a weapon that deals at least 1d8 damage.

The average DPR of this party against 14 AC is 46.95, while the average DPR of this party against 12 AC is 52.65, so, when the Wizard hits, it increases the DPR of the other members by at most 5.7. At most because this vulnerability may not last for the entire round, rather, it may actually last for absolutely no attacks from the party, if the Wizard is just before the Froghemoth in the initiative. So, if that is the case, and the spell you are using would deal less than 12 average damage, then even from a DPR perspective it is worth it.

This difference becomes slightly smaller if the party has ways to improve their attacks, such as advantage, bless, more bonus to hit (e.g. Archery), and becomes slightly better if the party has higher damage outputs, such as Rage from a Barbarian or a Sneak Attack from Rogue.

However, as you figured out, the turning point is actually decreasing the DPR of the froghemoth. Stopping the Froghemoth from potentially Grappling + Biting (+ Swallowing) all in one turn, or any other combination that leads to a character being eaten in one turn, helps a lot. The fact that it can't Tongue + Bite you is even better. Unless you are specifically playing some Evocation Wizard with focus in DPR, a Wizard is most likely controlling the battlefield, mitigating damage and crowd controlling. Let the optimized Warlock multiclassing into Sorcerer focus on dealing damage, the Wizard is more than happy nullifying the threat.

Your spell probably represents less than 25% of the damage of the party, but it may well represent more than 50% of the survivability of the party¹. Your party will never again complain that they are missing a Cleric.

Spell suggestions

Have you heard of... Witch Bolt? Honestly, if you hit, for the cost of 1 Spell Slot of 1st level, you are essentially making the enemy a lot easier. The cantrips mentioned in linkassassin's answer are also worth it, but you need to hit every turn, and, more important, they are (practically) melee range, which may not be where a Wizard wants to be.

Higher level spells are only a good idea if you want to guarantee hitting (some Save or half damage spell, for example). Remember: you are not trying to deal damage, you are trying to nullify the enemy's threat/damage.

Another way to see it: CR

The average damage (not "per round", but the way the DMG computes it: assuming everything hits) of the Froghemoth is 70 while not shocked (38 + 22 + 10), and 32 (22 + 10) if shocked. This reduces its offensive CR from 12 to 6 or 7. Its defensive CR is 7. So, when shocked, the monster goes from CR 10 to CR 7. Shocking is worth 3000 experience points.

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¹ I didn't actually do any math here: the Frog, without the Shock, attacks at least twice, while with the Shock it can only hit once. Careful analysis on the DPR of the Frog, including its bite and swallowing effects, may as well lead to a considerably larger damage mitigation.

Lightning is a lot better than no lightning

Let's say the party deals an average of 20 DPR per character, the wizard either deals 10 damage with lightning or 20 without.

Without using lightning the Froghemoth deals 69.5 damage per turn, and dies in 2.3 turns for a total of 160 damage.

With lightning, the Froghemoth deals 26.5 damage per turn and dies in 2.63 turns for a total of 70 damage.

Note this isn't exact. The Froghemoth doesn't have much AC, and lightning reduces the AC of the Froghemoth even further. Plus different party members have different AC, damage, etc. But you can clearly see that even from a cursory comparison there is a huge difference in the threat a Froghemoth poses when using or not using lightning.

Ultimately, halving the damage of 1 party member in order to reduce the enemy's DPR to around half is hugely beneficial.

A small amount of lightning damage is valuable

As @linksassin said in their answer, even a lightning damage cantrip is beneficial. If the wizard could inflict just 1 point of lightning damage per turn on average, that would mean the party takes just over 3 turns to kill the frog, and they take 80 damage. That's still huge, without having to burn high-level spells.

A small amount of lightning damage is valuable

Shock Susceptibility is a huge debuff to the creature, equivalent to the 3rd level spell Slow, though only single target and shorter duration. This is definitely worth sacrificing some damage for.

Should you use high-damage lightning-based spells to trigger this effect? Only if you don't have another way to trigger the effect. If you have a lower level effect, ideally a cantrip (Lightning Lure, Shocking Grasp), to trigger Shock Susceptibility and then save your higher-level slots for damage types they aren't resistant too.