Should I NP2 d'Eon

Higher NP doesn't mean it is great. This is especially true for support np like Jeanne anf Waver.

Personally I'd burn it. But if you're regularly using D'eon as a tank and staller, then you could probably keep it seeing as the ATK and DEF debuff doubles to 20% at NP2. Still, it only lasts 2 turns.

D'eon is not always appreciated as a debuffing taunter. I would NP2 this servant and make lancers tremble in fear as 20% attack + 20% def down debuff is effectively similar to 40% attack. Mine is max leveled and skilled up to lv6. D'eon needs Forbidden Pages and there is a long list of others who will get them first. You could burn the third copy. I burned my 4th copy of Tamamo Cat as a prism since getting her to NP5 is a very low priority for me despite being adorable. Therefore, I suggest burning a 4* after NP2.

Until I got my 5* sabers, my Rama and D'eon were in heavy use.

Of course, only burn 4*/5* when you already have the NP maxed. Unless it's Fionn.

At NP2 D'eon reduces the defense and attack of enemies by 20% with a tiny possibility to charm them. This is basically what 1* servant Mata Hari or a Waver can do with a skill. I think even at NP2 it's terrible.