If you are asserting that Tiamat has outpowered the tarrasque in every edition- which I think is what you mean, but your post is worded a little confusing- I have to say that you're flat-out wrong.
1e Tiamat: AC 0, hp 128, 6 attacks for 2d8/3d6/2d10/3d8/3d10/1d6 (average damage: 62).
1e tarrasque: AC -3, hp 300, 6 attacks for 1d12/1d12/2d12/5d10/1d10/1d10 (average damage: 64).
Granted, this includes neither Tiamat's breath and poison nor the tarrasque's sharpness bite and unkillability, but I think you have to admit that, if you're talking RAW, the tarrasque is traditionally as tough as Tiamat. The way 2e treated deities was an outlier in D&D's history; they had stats before 1e, in 1e, in 3e, in 4e and, from the looks of things, in 5e. I understand the position of those who prefer the statless, almost-limitless god, and I share the feeling to a certain extent, but that's atypical for D&D.