Yeah, probably.
Umbreon is really too slow to take advantage of Taunt, as setup sweepers will already have gotten their boost in. Also, consider replacing Curse with Protect if you wish to have a more stally Umbreon, which is generally what Toxic means: less power, more ragequits.
Umbreon@ Leftovers
Ability: Synchronize
EVs: 252 HP / 244 Def / 24 SDef
Relaxed Nature
- Moonlight
- Payback
- Toxic
- Curse/Protect