Sie hat zu wenig Zeit.
Er zeigt zu wenig Zuneigung.
This is the adverbial use of the pronoun wenig, and that's why it doesn't get adjective endings.
You can break down the grammar of these sentences as wenig is not a feature of die Zeit or die Zuneigung here, but a feature of the verb
(zu) wenig haben
(zu) wenig zeigen
Think of such common adverb-verb combinations as verbs of their own.
The adjective use of wenig works like this instead:
Es lag an der (zu) wenigen Zeit, die sie hatte.
The article der creates a brace with Zeit, which marks all the things inbetween belonging to der and Zeit. So it has to be an adjective here.
To make it even more complicated, even adjectives as groß can be used like adverbs:
Er hatte nicht groß Zeit.
This is groß in the meaning of viel.