Noob question #2 about Chevalier d’Eon

D'Eon will have a servant bonus in the next event, so levelling her might be a good idea.

Hmm, maybe no, he/she is a taunter, having a servant that cannot give a lot of damage ia a problem for newbie in this game and you already have Mash. Go for berseker until you have enough ember or item. If you don't have good berseker maybe try roll for Martha(ruler), Hercules,cu(fsn or proto),Nero or other high survival servant. They really help a lot for newbie

D'eon is worth leveling as he/she/it is a premium tanker and can protect your party. This should be done only after you have your main DPS servants leveled though. Early stages of story and majority of events do not require tanker.

Other answers are good. I will do more detailed:
Like Nero fest difficult quest in the near future, you will realize why these "taunter" are really important. Later on more and more challenge quest will be presented, if not archer enemy presented, "she" is really helpful.

Chevalier d’Eon is one best taunter. Even in comparison with Georgois, "her" star adsorption is mediocre which will not heavily affect your major critical DPS such as Raikou and Lancelot; and HP was much higher, as well as with better supportive NP and evade. Meanwhile in a good team, she can acted even more effective.
Like criticals team: Lancelot (both saber and berserker)/Raikou/Gil + her + Anderson/Waver, etc
Like normal NP spam art team: Vlad/Nero bride/Artoria archer/ + her + Tamamo, etc
Like normal Berserker DPS team: Cualter/Herc + her + Nightingale/Mashu/Waver/Medea Lily/Jeanne. or two DPSs if you want to less durable but more fast.

However, her shinning time is still not now, since her damage vs Lancers and berserkers are not satisfying, and very mediocre NP. You absolutely should invest SSRs and other servants with better damaging rather than her. But what about yours pointed Medea and Cu (pro).Cu (pro) and Medea are top-tiered 3 stars? They are also definitely worth investing.
But.....
Considering if they have better replacement as SSR and SR now or in the future, they do. Now Cu (Pro)'s tank ability is decent but surpassed by Cu FSN, after strengthening he can deal decent critical damage, but could this critical damage as good as Karna, Brynhildr, Artoria Lancer Alter or Raikou Lancer at almost the same time for Cu pro's skill upgrade?
Same for Medea, indeed her NP effect is really valuable and capable to spam, but is it necessary to get her 10/10/10 and grailed if you want her worked as a major DPS to Assassins and when you already have Xuanzhang, Nursery Rhyme and Da Vinci? Maybe or maybe not.

However you Cannot find Any SSR and SR with excellent Combo of Taunter as Chevalier: self healing+{3} turns taunt+self debuff removal and 115% resistant up (again, {3} turns)+evade and defense up + debuff NP damage (though charm successful rate is low, no gender or enemy type required and AOE).

My personal suggestion is depending on how you will spent money and time on this game. If you want to have more SSRs, you dont have to invest everything for SRs and 3 stars. If you are a FTP, Cu Pro and Medea are more required to be invested. But no matter what ways, you can at least level her to 3rd acension and slowly increase her skill 3=1>2. After next year her strengthening, try to further invest her.
Meanwhile don't forgot her bond CE. Just imagine a team of Nero (both two) + her + Tamamo with all three bond CEs.