Homebrew PCBs

You want the toner hot enough to get "plastic" and sticky.  Too hot
and it just melts, which is probably what's happening in your case
(assuming the print itself is good).

You can experiment with iron heat using a laser transparency instead
of paper.  Print on it, set your iron at a lower heat, and see what
happens.  Keep increasing the iron heat a little at a time, and keep
notes about which settings stick or don't stick, and deform (melt) or
don't.  You want a setting halfway between "just hot enough to stick"
and "just hot enough to melt".

Me, I ended up getting a laminator.