The thing with Mango is that he's consistently inconsistent. During the fall and winter, he doesn't try as hard and drops many more sets to Gods. However, once Summer starts, he starts to dominate so hard that he's undeniably the GOAT.

There's a phenomenon called "Try Hard Mango" who only comes out when it really matters. At WHOBO, Mango loses to Hax in Winner's Finals, and then comes back as Try Hard Mango to 6-0 him in 2 sets. Similar things happened at the Big House 4. Mango went into the tournament not trying because he was relaxing from his wins in the Summer, but went try hard with Captain Falcon to humiliate Hax to show him that Falcon was a usable character. Lucky, Mango's best friend, also performed really well at that tournament, causing Mango to also want to try and win the tournament for him. Mango destroys Armada while wearing the "Joey" tag, which is Lucky's real name. Then, he beats Leffen even worse including a four stock in the last game. In Grand Finals, he manages to clobber Mew2king, even on Final Destination where Mew2king is know to thrive on! And this is after M2K already 3-0d him in Winner's. Case in point, not trying Mango loses to Mew2king, Try Hard Mango beats everyone.

Now, as for whether Mango really is the best, he has a rabid fanbase who tend to dickride him and constantly talk him up, so the amount of people calling him the best may be skewed. That being said, Mango clearly is one of the best players, and has the potential to beat everyone when he's trying his hardest.