First off, Mike Williams is 6'3"+ almost 6'4" so dont know where youre getting 6'1" from.
Second.... yea its a transition for an offense like that to the NFL in terms of learning a playbook; but its usually knowing/perfecting the full route tree for receivers thats the issue. Mike Williams can be given a dumbed down, simplified version of routes and just go win.
Mike Evans, Sammy Watkins, DeAndre Hopkins (both Clemson guys), Tyler Lockett, Brandin Cooks (almost purely screens and vertical routes), Keenan Allen, Dez Bryant, Alshon Jeffery, AJ Green, Calvin Johnson, Demeryius Thomas, Michael Crabtree, etc...
All guys taken pretty early in drafts that were either size/speed box out/jump ball type guys, played in very simple offenses for WR's that didnt require complex route running or trees, or some combination of the two that have been very very good to dominant in the NFL.
Two of the most recent hailing from that same Clemson offense. Watkins was certainly faster but smaller, lesser catch radius, and didnt work the middle of the field... and Hopkins while a better route runner for sure was also much smaller and slower.
Not saying this guarantees him success... but plenty of guys with the same skill sets have become great receivers recently, and others with some of the same skills/traits and from similar or the same exact spread/gimmicky offenses have transitioned fine to the NFL as well. In fact, almost half or more of the best WR's to come out recently fall into this category. List of the 10 best WR's in the NFL and i bet more than 5 have similar traits, similar concerns heading into the draft, and/or came from similar offenses in college.
Definitely not a reason to write him off when you see how he was able to raise his game against the best competition.