yes 'ridiculous' is gradable as you define it.
His clothing was extremely, utterly, disgracefully, amazingly, and ridiculously ridiculous! :D
however I tried to answer your main question, I just couldn't come up with an example of an adjective that [i]couldn't[i] be modified as such! could you please give me an example of an 'ungradable' adjective? Can somebody?
Some, like 'dead,' 'married,' 'pregnant,' etc. are logically not used with these modifiers because they represent a state that has no in-betweens. I suppose that formally it would be wrong to say "very dead" or "kind of pregnant" - but I have to tell you that these are famously funny jokes for just that reason. There is no grammar written in stone (and hence no rule) about which adjectives can or cannot be modified by a word specifying some degree of the effect.
So if you want to know the answer for any particular word, think logically about if the word has possible degrees of effect:
You are either dead or you aren't, there is no in-between
you are either pregnant or not, again no 'halfway'
you are either present or absent, no percentage thereof.
And remember, you can use one of these 'extreme' adjectives with a modifier of degree for comic effect or to emphasize your point.
example (emphasis): "the thought of death was very present in my mind"
example (comedy): "He is just a little bit dead"
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