A failure would be too much criticism. He is leading the goal scorer list and I think as long as he is the best scorer, it can't be called a failure. But I think the better word here is that he did not fulfill expecations and this is most relevant for the highly important games. It is not that important that Mbappe scores against Leganes or Valladolid. Real Madrid made a big move to sign him because they want him to score against Barcelona and the teams in the CL when it is all about that one goal at the right time. This has only sometimes worked out, but definitely not as often as the club and the fans were hoping for.
Criticism is fair because it may not be in line with expectations but leading the goalscoring charts this season for the team is a good achievement even though overall he has failed to bring Real Madrid the title. He also managed to score against Barcelona even though they ended up losing and that defeat was also not caused by Mbappe but rather the defense which made too many mistakes. Mbappe cannot do it alone without the support of a solid team and even his predecessors were supported by a solid team that managed to achieve great success.
Mbappe is missing teammates that relentlessly find the deep gaps that Mbappe can use to exploit his speed advantage. They need a top class midfielder again who understands like nobody else when to play that pass and where Mbappe will be going without even seeing where he is. Sometimes that takes time in a team or, alternatively, genius. When Messi and Mbappe played in one team, Messi was a guy who didn't have to see all the time where the forwards are. He knew best how to play that ball because he was a forward himself while also having strong midfielder abilities.
Mbappe can become the next 40 La Liga goals a season player if he has a solid team around him.
As you mentioned the weak defense by Real Madrid, Barcelona wasn't doing very well in their defense either in many games, but their strikers made up for it.
Nevertheless, this was a season with great clasicos again. Finally some games with so many goals. That's when the clasicos get interesting.