Hoy hace mucho calor.
I see that the translation for it is
Today is very hot.
But I think that sentence is very complex and difficult to language learners like me. So can we simply write
Hoy está mucho calor.
You must use
You can't micromanage a target language like that,.. "..that's just too confusing, I'll say it this way"
Está would not improve your sentence nor would it be considered to mean the same thing. In fact, it is grammatically incorrect
Hoy está mucho calor
Calor is a noun, which means heat. So you would effectively be saying...
Today is very heat.
Your direct translation is not completely accurate. "Today is very hot" literally means
Hoy está muy caluroso.
Since a direct translation of "Hoy hace mucho calor" should be "Today there is very much heat" (obviously wrong), your attitude as a translator should be such that indicates by Kent Anderson in his answer.
"Hoy está mucho calor" is completely wrong. Alternatively you can use:
Está haciendo mucho calor. (*)
Está siendo un día muy caluroso.
(En estas dos frases, el gerundio hace innecesario especificar que se refieren a «hoy», y por tanto se suele omitir.)
Hoy es un día muy caluroso.
But this is my preferred expression:
¡Ozú, que caló!
(This is Andalusian dialect, but any Spanish-speaking knows what it mean
(*) Note that "mucha calor" is also correct. (you can use "la calor" o "el calor").