Although some took the use of mixed-case font in House of X and Powers of X to possibly indicate a connection to Ultimate X-Men, writer Jonathan Hickman says that isn't the case.
During an interview with AiPT!, Hickman was asked whether or not his use of the mixed-case font was meant as a nod to the Ultimate X-Men, which used the lettering style. "We ended up using Helvetica Now and Helvetica Monospace for our design fonts," he said. "These were not Tom’s original choices (I think those were Replica and Akkurat Mono), but it’s where we ended up... As for why we use a mixed-case font instead of an all caps font for our dialogue, sorry to disappoint you, but there is no story reason for that, it’s purely practical. I noticed a couple of years ago that you could stick more words in a smaller balloon if you used a mixed-case font and so I made it standard for all my books. I like looking at a page and seeing the finished art not the word balloons. FIGHT ME."
Earth-1610's version of the X-Men made their debut with Mark Millar's Ultimate X-Men #1 in 2001. The series launched shortly after Marvel introduced its Ultimate line of comics, which was a modern reimagining of many of the publisher's classic characters.
X-Men #1, by Jonathan Hickman and Leinil Francis Yu, is available now.