As stated here,
...rationality is the characteristic of any action, belief, or desire,
that makes their choice a necessity.
So, attributing rationality to the universe, means that somehow the universe needed to favor some actions to take place. But at least with humand kind, there is no proof that the universe needs humanity at all.
Also, if rationality of the human intellect were a reflection of a rational universe, then every human action attributable to rationality, should be a reflection of the nececities of the universe. But again actions taken by humanity have no consecuense for the universe.
Aquinas' argument could be blamed on being anthropocentric, as he is attributing a very optimistic human qualifier to the universe itself, as is rationality.
Pretty much in the same way, life and death are qualifiers that could be applied too, even when they are contradictory.
If we try to be fair, we have to give those qualifiers only to region of the universe that has been influenced by human kind. So, we need to consider for instance the volume within the region covered by the first human-made electromagnetic emissions, compared with the region outside that volume. In wich case we could tell that the universe is only negligibly rational.
As there is no such thing as a rational universe, Aquinas' argument is false.
Even when it could be true that if there were a rational universe, it's because of there could exsist a creator.
Is the universe rational?
Aquinas says yes, it is rational because we are rational.
We use the word rational only to describe human actions, or thoughts.
Let's extend that use not only to describe humans, their actions and thoughts, but also everything we have influenced or altered somehow.
Now, what region of the universe is actualy human, and thus rational.
I have to say that only a negligible region - even considering that our influence is moving across the universe at the speed of light, with our TV and radio broadcasts.
How can someone say that the whole "thing" has some "caracteristic", when that "caracteristic" is observed only in a negligible part of that "thing".
It seems to me more fair to tell that the universe is not rational at all.