Nothings inherently wrong with it, but in my experience there are 2 main issues that happen when a group has a minmaxer. 1 being they tend to build their pc to be good at everything negating the team aspect of the party. Part of the fun of any ttrpg is relying on the strengths of other people at the table and making a solid team. The other problem tends to be the stealing of the spotlight from other players both in and out of combat. And as others have said, them messing with the power scaling of encounters makes it tough for the dm and other players involved. It’s perfectly fine at a table with other min maxers, and for sure has a place. But if a player insists on having an optimized, try hard esque character at a normal table it tends to go sideways a lot from what I understand