You could...but you probably shouldn't. And it may not really matter anyway.
There are a couple of things to consider when determining the possibility of this tactic. The first is whether or not you can get inside the oral cavity of a Tarrasque. This thing is huge, so using Reduce in order to do so seems functionally possible.
The problem is that there really isn't any sort of mechanics for going inside creatures. The bigger issue is likely in entering the Tarrasque in the first place - especially if you go in through the mouth.
The Tarrasque has both a Bite and Swallow attack. If you're inside it's mouth, it's very likely going to use that against you. THe other is the secondary part of the Swallow attack that concerns me more about going inside this dreaded monstrosity:
While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque's turns.
Being inside, or at least in the stomach, seems like a particularly dangerous place.
In addition, and more importantly, 5e doesn't have a called shot mechanic. Attacks are much more generalized. What you may consider is reducing the AC, but even that doesn't happen to the Swallowed creature, so I'm not sure there's a precedent for that and would suggest keeping the AC the same.
Getting up there
You should also consider how they're getting into that mouth. This thing is 50' tall, they're going to need to fly up there somehow.
Rule of Cool
However, this is a pretty cool idea. As a DM, you need to balance cool ideas against powerful creatures - and the Tarrasque is one of the most powerful. If you're saying this is a viable tactic, allow called shots, etc. - then you're significantly reducing it's CR as well.
The problem is that once inside the mouth, they are still a candidate for the Tarrasque's Bite and Swallow attacks. I'd also have to heavily consider that if they're already in the mouth/throat of the Tarrasque, the Swallow is likely just going to happen. They don't need to bite them first to get them in their mouth.
It's likely not the best idea functionally even if it sounds cool at first pass.