Damage Up isn't essential for the Sploosh, but a few subs of the ability wouldn't hurt (though I do consider 1 main and 3 subs to be excessive). As high as the Sploosh's rate of fire may be, its accuracy and range are abysmal, so having to land an extra shot to secure the splat can cause problems.
three defense up mains or more is really uncommon
Correction: It
used to be really uncommon. Nowadays, however, Japan seems to have a thing for just that sort of build, particularly if they can pair it with Ink Resistance. As Tim H. mentioned, it's most common on the .96 Gal, but I've seen a few other types of players running Defense Up as well.
Anyone with that much Defense Up is handicapping themselves in other significant ways, so I wouldn't worry about countering them in the first place.
I disagree. Defense Up stacking isn't a good strategy for every weapon, but a blanket dismissal of it isn't appropriate either. The .96 Deco in particular tends to get the Defense Up treatment because, as a frontline defender with an amazing defensive sub (the Splash Wall), guaranteed 2HKO capability, and awful strafing speed, the .96 Deco has no need for Damage Up and tends to get less use out of abilities such as Cold Blooded and Run Speed Up than, say, a Tentatek, which is much more reliant on stealth and agility. With Defense Up, the .96 Deco user is less likely to drop Kraken, and can turn what would be trades with Tentateks and .52's into wins.
Now, granted, Defense Up isn't the only way to go with a .96; you could focus on Ink Saver Main or Swim Speed Up instead. However, I definitely feel that Defense Up is one of the go-to options on the .96 Deco if you have the gear for it.