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.