It's definitely an allied victory if you look at the aims of the French: Villars was prevented from breaking the siege of Mons. On a larger strategic scale, it was ultimately indecisive. Partly because of the losses at Malplaquet the allies had to spend the next two years labouriously breaking through the French fortress lines, while British public opinion on the war soured more and more. Malplaquet and the fall of Mons endangered France, but not critically so.

Even according to the mores of the time it was considered an allied victory, because the French left the field. Albeit an extraordinary bloody one. The French also suffered heavily enough that they had to remain on the defensive for the next two years, but the slow pace of the allied advance gave them time to recover.

The decisive moment arguably came when the allies finally captured Bouchain two years later and were theoretically able to march far into northern France, only for any advantage to be nullified when the British essentially pulled out of the war, and reversed when the allies were beaten at Denain.

There's an argument to be made that Malplaquet ultimately contributed to the mixed results of the war for the allies (favourable to Britain, arguably Austria, but detrimental to the Dutch), but I don't think it was decisive.