Could I build a question just putting the verb first?
Finns det mer mat?= Is there more food?
Certainly, I would say that here "more" is an adjective describing "food". Sometimes it is an adverb, when it qualifies an adjective or another adverb - more important or more quickly.
That sentence would rather translate to "det är mer mat". Here, it's "det finns", which in this sentence translates to "there are/is".
mer is quantity and fler is specific or typical (depends on context).....more food vs. several types of food