Yes, it was one of the worst bombing raids and yes, we would consider it a war crime (or at least extremely unethical) nowadays (much like many things both sides had done throughout either World War) but it's usually invoked to play down nazi crimes and it's usually misrepresented because of very successful propaganda.
The estimated death toll was around 25-30 thousand. During and after the war various revisionists and propagandists have blown up that figure to over 100 thousand. Additionally German nazis have built an entire tradition around it, "mourning the fallen" to exploit the incident as a display of nationalism and nazi sympathies.
There's no legal basis to single out the bombing of Dresden as a war crime within the context of the Second World War. I would say that it can be argued that some of the crimes nazis were charged with after WW2 also lacked a pre-existing legal basis but there was no expectation that the winning side would apply punishments consistently, sufficiently or even fairly.
So even if there were good reasons to categorise the Dresden bombings as a war crime in the context of WW2, it'd just be one of many alleged and proven war crimes that went unchallenged -- including other crimes (and "crimes") committed by the US, the Allies in general or any other involved military.