Thanks for reaching out. We understand your concern.
Currently, we can only export in the formats mentioned above with Lightroom mobile. To convert the images into PNG, you need to use any other application capable of doing it (Example: Photoshop).
It is not currently an option, but I personally like PNG too for certain situations. I will move your question to the Ideas section of our forums, so it can be counted among other enhancement ideas
After scratching my head looking for solution on the web I had to do this,
Kindly give us a feature or option where we can save a image as .png file in lr mobile.
Technically .png file holds more colours than jpg or any other format therefore it gives us more clarity and crisp colours and I'd like to request the team to think about it.
I could export in PNG because it would help to have a file with more quality when posting on instagram stories. I'm having to edit on the computer to get.