The character designs always remained silly. There was a never a point where Oda's character designs became serious or anything. Thats not to say there were not characters who were serious looking, but thats always been the case. Oda as an artist just became more angular and his linework a bit rougher, and I think you could very much put it down to both him evolving his style as an artist, but also the schedule forcing that evolution as he got older. Its not a secret that Oda jams a lot of stuff into chapters among the fact that Oda has shown that he is a naturally more sketchy artist(it probably works easier for the work flow to have a more sketchy final product as well).
There are a few reasons why its better. The art style optimises line work, allowing the characters to be more animated and visually interesting. More detailed designs with greater shading or linework can look fine but they are more difficult to animate in motion. For a series like One Piece where a massive appeal of the characters is their massive personalities and exaggerated expressions, having the ability to more easily animate the characters works wonderfully in capturing the type of human and characterization that is necessary to make the characters look fun on screen. Character acting animation is something that I think One Piece would definitely benefit from and the new sleaker character designs seem set to making character acting animation more attractive.
And it also helps when it comes to other types of things like action scenes.
I think I am making pretty reasonable assumptions about some things here in terms of how Oda has evolved as an artist. His current art style is probably not necessarily intentional as much as it is "how his art has evolved given the circumstances he is in".
Again, this is somewhat opinion, but I think I've backed up what I've said with the reasoning stated above.