If you aren't aware of the floating island this guide refers to, it's extremely easy to navigate to. Just use trampolines and your glider (press the jump button while in mid-air) to ascend until you reach the island that has two pairs of arcade machines and two houses, one with a beautiful chimney (your target).

Here's a screenshot of the island from a chimney's eye perspective:


Additionally, here's a screenshot of the archipelago with a few lines showing an easy route you can take to reach the island:

There is an easier way to reach the chimney by gliding using the trampoline shown in the bottom of the above image, but that's boring. This guide is about climbing the house, not just reaching the chimney.

First, here's a beautiful MS Paint rendition of the house and the route I use to climb it (which I currently believe is the shortest, but please tell me if you find a shorter route):


The action you need to do is color-coded (e.g. a yellow line that turns green means to jump then dive mid-air).

Here's a few tips on how to navigate the house, each with a respective "*" on the above image:

• Tip 1: There's more than one way to scale these windows. You can usually jump or jump+dive onto them from the side or the front.
  
• Tip 2: You can reach the other side from the lower section of the roof here, but you can skip it with a well-timed mid-air dive (solid line).
  
• Tip 3: You have to line up your character almost perfectly facing the spire to make this jump. I recommend placing the camera directly overhead.
  
• Tip 4: You need to dive slightly earlier than it feels like you should.

Overall, the hardest part is definitely the glide from the spire to the roof with the chimney. It took me dozens of tries the first time I tried this during the beta and somewhere around 7-10 the second time. To maximize your chances of making the jump, here are a few extra tips:

• Jump forward and start gliding almost immediately for best air-time
  
• Try to aim straight for the low part of the roof so you don't have to adjust your angle mid-flight, which loses some time.
  
• It is possible to overshoot the jump, which happened to me once during the beta!

Once you complete the spire jump, the chimney is pretty much yours. When jumping from the high section of the roof to the chimney, I recommend jumping towards the part of the chimney close to the roof ridge rather than the center so that if you miss you can try again.

Good job! You are now one of the microscopic proportion of the human race that has reached the highest point in the Granblue Fantasy Versus: Rising online lobby the hard but very cool way! Your reward is the ability to brag to your friends about it and feel good about yourself. For more satisfaction, pick an avatar with a funny unique action (personally, my favorite is Lowain's) and do it on top of the chimney.

If just climbing the house isn't enough for you, feel free to go further than I did and try discovering new routes or maybe speedrunning the chimney. (GBVSR Chimney% when?)

Additionally, it doesn't seem to be the highest point in the lobby, only the highest reachable one. There's also that island far in the distance, placed seemingly to taunt anyone who reaches the chimney with the tantalizing pull of unachievable greatness beyond thine own.
I don't know if it's possible to reach that island legitimately. But mankind has breached the skies before and planted its foot on the soil of aetherial rocks far higher. Your only limitation is your own desire to achieve the impossible. Reach your hand out, and touch the face of God.