If Tata Martino didn't believe it, he wouldn't say it.
The recently appointed Mexico national team coach is a man of his word. And Martino not only has he said it once, he's said it on multiple occasions: The Argentine isn't so sure European leagues are more competitive than the leagues in North America.
It's important to note Martino isn't saying the quality is higher in MLS or Liga MX than in La Liga or the Premier League. He's talking about parity. Teams in MLS and Liga MX are much more level. Grandes like America win championships, but so do less-storied teams like Santos Laguna.