I encountered the same issue. Setting initial zoom didn’t fix it.
Trying to make my own workaround I discovered that this blurring was occurring in the img-tag as wel. I found out that images with a resolution from around 5500x3500 pixels are blurred anyway, also in a simple img tag
Open this link on your phone: http://dog3.nl/test/mobileResolutionTest.html
On my S4 from image 5 the images are blurred.
I now simply resized my images to 5000x3300 and it al works fine. Offcourse, only because it’s still big enough for my purposes.
I only tested it on a Samsumg S4, so I have no idea wether an image 5000x3300 shows well on every phone or not.
But we can be sure it is nog an ionic-issue.