As far as I know, we say that the words in which every letter is in upper-case as "ALL CAPS".
Then how can we say for not "ALL CAPS"?
Something like "Not all caps".
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We would usually say something like:
All lower case.
There isn't a direct opposite, most likely because "CAPSLOCK" is a thing, and its opposite would just be the absence of that thing, as opposed to another thing.
You may also mean "proper case", which is where the first letter of each word is capitalised, e.g.,:
This Is Proper Case
For something like "Not all caps", you may be looking for "sentence case".
This is sentence case.
There is so-called "sentence case" (where only proper nouns and the first letter of the sentence is capitalised.
There Is Title Case in which All Words Except Particles Are Capitalised
and then there is lowercase in which no words are capitalised.
We rarely use the expression "sentence case" because this is the default. All writing should be in sentence case (except when there is a specific reason otherwise)