No. "Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before. You can use this feature twice. You regain expended uses when you finish a short or long rest... At 2nd level, for example, you can transform into any beast that has a challenge rating of 1/4 or lower that doesn’t have a flying or swimming speed."
The Rules As Written say nothing about needing "a stat block" before you can transform into a creature.
Turn the page. "Your game statistics are replaced by the statistics of the beast"
A rot grub doesn't have statistics, not having a stat block. Since you cannot replace your statistics you cannot become a rot grub.
It's almost as if you've never seen the parts where Godzilla gets attacked between cities by the Japanese Defense Force or the American military. It's not like he's stealthy.
If the PCs are 2nd level (or 5th level, or whatever) and they have to deal with the Tarrasque, you can bet they should be ambushing it somewhere other than the middle of a populated city! It's almost like we have different levels of railroading in our respective campaigns...
Since I'm not railroading, I wouldn't force my level 2 or even a level 5 party to fight a tarrasque. The story then is "fleeing the tarrasque" or maybe "evacuating people".
But if my party at that level decides to pick a fight with the tarrasque, I'm also not going to make it easy on them by having the tarrasque decide to fight in a big featureless plain devoid of innocents. Some comically remote and uninhabited region of the world where there are no stakes and they can kite it to death. This is D&D, not DragonBall Z.
So the druid decides to fight the tarrasque by turning into a rot grub. Let's go through the scenario.
First, the DM needs to give the rot grub statistics: ability scores, hit points, an AC, etc. basically write a whole new statblock.
Rot grubs gave a reach of 0, so the druid need to use *almost* all it's movement to get close. Which means it needs to be really close, but not so close that the tarrasque can close the gap. Or just walk away from the grub crawling away at 5 feet. Very precise situation.
But, for the sake of argument, what happens? The druid moves right up to the tarrasque and gets within 5 feet. Then changes and moves the final 5 feet into the space of the tarrasque. And attacks. With a +0 to hit, the grub needs to roll a 20. So there's a 95% chance it misses. The tarrasque then gets a legendary action. And likely a turn before the druid goes again. It has a 95% chance of hitting the AC 8 rot grub and doing more than the 1-2 hp needed to return the druid to human form. Very likely it rips the druid apart.
But what if the druid does hit? Well, the rot grub is making a melee weapon attack to hit, and the tarrasque is immune to all damage from nonmagical sources. So the grub wriggles away at the skin, unable to burrow down.
Now, if the DM isn't using the errata and rules that this rot grub damage is able to hurt the tarrasque, it does 1d6 damage each round. So 3.5 damage each time the tarrasque starts a turn. 225 turns later it dies.
But it can get pretty darn far during that time.
Or, yeah, you could have the party buy rot grubs and throw them individually at the tarrasque, which counts as "coming into contact". If they hit with more than three then the tarrasque can't use a legendary resistance.
But at that point you might as well just give the party infinite experience and stop playing. After all, that tactic works on every creature with less than a +9 to Dexterity saving throws. Or the DM reserves the right to have the monsters start doing the same thing...
