Just to be clear the articles I read didn't specifically mention deck heating as an issue - just that the JSDF designed them for a small number of helicopters.
"Hyuga" carries a max of 10 x SH-60 and has landing spots for 4, the new 22DDH's will carry up to 14 helicopters but will normally embark 7 x ASW + 2 x Minesweeping/transport helicopters on 5 spots
Presumably modding them to be able to fully operate, rather than just demonstrate F-35 landing & take-off on a single spot, is just not attractive
It will be interesting to see if the Japanese decide to order more DDH's with increased capacity in the near future but their current emphsis is to buy more DD-115 class destroyers I believe