Ramps aren't inherently bad, but generally aren't necessarily the ideal of what you're going for.
Obviously, CATOBAR is the gold standard. Everyone wants to be them. However, they're expensive, traditionally maintenance intensive, and take up a lot of internal space. That being said, the advantages they provide are usually worth it- namely the ability to launch heavy aircraft. This is especially noted not in fighters, but in heavier strike aircraft and support aircraft: AEW-C, EW, tankers, and the like. If you're going to be building a purpose built carrier that is intended for much of anything but providing fleet air defense with its fighters, you should be attempting to use CATOBAR.
Just a straight flat top is obviously the easiest and cheapest, and leaves the deck space available for helicopters, something which is a big consideration for the US with their LHDs and LHAs and (currently) Japan with their DDHs. Their primary role is not as small aircraft carriers, but rather as platforms for helicopters to take off from either to land Marines elsewhere or as ASW platforms, respectively. They can be used to launch STOVL aircraft, but aren't ideal for it. Fixed wing support assets can't take off from these.
Ramps, on the other hand, allow for aircraft to launch with a higher weight than just a flat top normally would, however most fixed wing support assets can't use them. They don't take up any internal room, and so, if necessary, could easily be retrofitted onto an existing flat top to make it more suitable for flying STOVL aircraft off of. However, it's losing out on deckspace for helicopters.
So, with that out of the way, if Japan were to decide that it wanted a ramp on some of its DDHs, it wouldn't really be a bad idea. I'd disagree with it, though, as I perceive their greatest strategic threat to be submarines. Aircraft operating out of Japan itself can already support 99% of Japan's areas of interest, but there's never enough ASW to go around.