What do Q# rankings say about journals? As you know there exists a journal ranking namely, Scimago Journals and Country Ranking. And we see Q1, Q2, Q3, Q4 rankings. My question is that whether this ranking is a valid criteria of journal rankings in the field of mathematics?
Is it valid in the sense that it is likely roughly correlated with the prestige of a journal? Probably. Does anyone actually use the Scimago rankings? Not that I'm aware of. There's a general consensus about what the very top journals are in mathematics with a general consensus that Journal of the American Mathematical Society, Inventiones, and Annals of Mathematics, and Acta Mathematica (not to be confused with Acta Arithmetica) are probably within the top 5 or so, and Annals is probably the most prestigious. 

Curiously though, looking at the Scimago rankings, the Annals isn't listed in the top 50 even, and while they have Inventiones and JAMS near the top, their top 50 also includes journals I've never heard of in specific subfields like "Swarm and Evolutionary Computation". They also don't include some decent journals at all on their list. So the upshot is I wouldn't read too much into it. If a journal is in Q1 it probably is a better journal than if it is listed in Q4, but even that doesn't seem conclusive. Talking to mathematicians is probably a better way of getting information about how prestigious a journal is than paying attention to these rankings. And certainly mathematicians don't normally talk about or pay attention to the Scimago rankings.