He’s accomplished more but it’s a weird argument as we’re looking at Mbappe as a young player and trying to gauge his potential while we are looking at Messi as a young player and we know what his potential was.
So it’s difficult to separate that, you can look at what Mbappe has achieved and say “see Mbappe was better look at the stats” or you can look at Messi and say “we know how talented he was, you can’t compare his journey to the first team with Mbappe’s when Mbappe played at Monaco and then PSG in Ligue 1, while Messi was behind a number of stars at Barcelona in La Liga“. In all likelihood, given the same career path, Messi probably wouldn’t have been far off his accomplishments but we will never know that so all in all it’s a pointless argument.