There's that, and Slimify has its uses as well (but it's not fantastic). There's also an innate "cure mutation" ability at high levels that you can activate that only targets "bad" mutations, and at max piety you get the mildly useful "jelly shield", which causes random high-level slimes to spawn whenever you suffer a particularly nasty hit.
You also become naturally nonthreatening to all slimes and eyes. Furthermore, you can sometimes get jellies to freeze in their tracks for a few moments so you can snag some item they were about to munch on. However, this carries a surprisingly large piety cost, and worshiping Jiyva will give you a reason to use this more often than otherwise (a neutral red jelly will naturally spawn on every level you occupy so it can start Hoovering up all the dungeon-floor detritus). Piety is relatively simple to gain though, but it doesn't go up as quickly as you might hope.
All in all, Jiyva is the go-to god if you want to have some seriously nice mutation sets. You'll get your ever-changing pack of Jiyva-granted muts, but you'll also have the mutations that you acquire through other means (so far as I know, Jiyva leaves these alone). With the way the Cure Mutation ability works, you can freely chow down on all sorts of mutagenic stuff and then filter out the bad stuff afterwards.
...but that's really all Jiyva is good for. Don't count on any of the other abilities to get you through the rough times.