I'm an amateur at Ancient Hebrew but what the OP raises is a classic difficulty with translations.
rō‘eh ’êp̄er, lêḇ hūṯal = "Feeding on ashes, the heart is deceived" (YLT)
The Hebrew phrase is explained in the commentaries. https://biblehub.com/commentaries/isaiah/44-20.htm. It's to do with the foolishness and futility of trying to eat ash. It's metaphorical language - an adage or saying.
NLT renders the bible in contemporary English. https://en.wikipedia.org/wiki/New_Living_Translation
On the one hand translators want to keep as close to the original idea as possible. But "He feedeth on ashes" isn't something we say for this. There isn't a direct equivalent but NLT's "poor fool" is at least universally-understood and very neutral, e.g. compared with a closer phrase like "dumb enough to eat rocks" which is a dialect translation that wouldn't carry equally into all English milieux.
This is a least-worst strategy of translation: to get across the basic point without introducing new or specifically English ideas, but it loses whatever tone or subtlety the original idiom conjured up.
It's never safe to draw conclusions from the translated word. "Poor" might be appropriate in the English idiom "poor fool" but that doesn't let us conclude materially poor or any of the other ideas we derive in English. We have to remember we're reading a translation. So it's only poor in the sense of poor fool, and only here in this instance. The same Hebrew term might need a different English phrase next time it's used.
Re. Matthew 5:3, again no - that's coming from the placeholders of a translation.
In Isaiah 44, there might be a sense of spiritual poverty or even pity being displayed toward the idol-worshipper, but this is the self-created poverty of someone who has squandered material resources on an inert statue whilst neglecting the living god. And they're going to be contrasted with Jacob's and Israel's Jerusalem.
Comparison to Matthew 5:3 might be productive, but it doesn't arise from the word "poor" which is only a translator's gloss.