In uncountable|lang=en terms the difference between ridiculousness and silliness
is that ridiculousness is (uncountable) the characteristic of being ridiculous while silliness is (uncountable) that which is perceived as silly or frivolous.
In countable|lang=en terms the difference between ridiculousness and silliness
is that ridiculousness is (countable) the result of being ridiculous while silliness is (countable) an act that is silly; a result of being silly.
As nouns the difference between ridiculousness and silliness
is that ridiculousness is (uncountable) the characteristic of being ridiculous while silliness is (uncountable) that which is perceived as silly or frivolous.
ridiculousness
English
Noun
(uncountable) The characteristic of being ridiculous.
(countable) The result of being ridiculous.
Synonyms
* ridiculosity (less common)
silliness
English
Noun
(uncountable) That which is perceived as silly or frivolous.
(countable) An act that is silly; a result of being silly.