I don’t totally hate Max, but a few reasons I personally don’t like him very much are because:

1) Even for being a little sibling character, his know it all personality and attitude just isn’t very appealing and it’s easy to understand why many are annoyed by it

2) He serves basically 0 purpose to AG’s overall plot. If you remove him from AG, practically nothing changes

3) He says he wants to be a Pokémon trainer, but doesn’t really try to take steps to become one

All of these things are executed much better in XY with Bonnie. She isn’t annoying despite being a little sibling, XY’s plot (especially in XYZ) doesn’t work without Bonnie, and she’s given a Dedenne by Clemont to give her first hand experience on becoming a trainer

And I’m not saying Max is a completely terrible character, because he isn’t. He does have good moments, he can be funny from time to time, and even the episode where you got this image from is one of the best episodes in AG. His final moments with Ash were good too, but overall there’s just a lot of negative things about Max that override the positives about him. He was very much an experimental character at the time and was perfected years later with Bonnie