As the title says, mob heads don't get affected by any hud setting you might have.
To replicate:
-After getting any mob head, turn you hud opacity to the lowest setting available. Everything will become transparent except for the mob heads
MCPE-28721
HUD opacity doesn't affect mob heads and banners
Resolved
Fixed
2
(Unassigned)
90818
Confirmed
Windows
Windows 10
1.2.3.6, 1.2.6.2, 1.2.10.2
1.2.13.54
Retrieved
This bug has been reported to our internal bug tracker for further testing and a fix. It is scheduled to be fixed in one of the future updates (no specific date can be provided).