Yae Miko, A.K.A. "the better Fischl", (that is a joke) is an excellent five-star electro catalyst wielding on-field and off-field damage dealer. She is often overlooked and undervalued because of people's tendency to compare Yae with the four-star Fischl. Although some believe Yae Miko is not worth the primogems because of Fischl's free-to-play friendly aspect which is a valid opinion, I believe Yae Miko is still a great pull for her versatility, great AOE and single-target damage and electro application, and ease of build.
Why should/shouldn't you build Yae Miko?
Even if you already own a built c6 Fischl, Yae Miko is still not a bad pull! In fact, it might be a very good upgrade for your account because Yae Miko and Fischl synergizes well.
Both Fischl and Yae Miko's kit are perfect for this team. Yae Miko becomes a great Fischl driver by consistently being able to proc Fischl's A4 passive even off-field! Also, Yae's (sort of) hybrid damage scaling of attack and elemental mastery works wonderfully for aggravate. With Kazuha's viridescent venerer shred, elemental damage bonus buff, and Nahida's elemental mastery buff, both Yae Miko and Fischl will deal a great deal of damage. Not to mention the entire team's energy requirement significantly lowers. Fischl and Yae Miko generate particles every second with practically 100% uptime. Additionally, the electro resonance will supply even more particles.
Also, compared to Fischl, I would argue that Yae Miko is generally better as an off-field damage dealer. Yae Miko deals higher raw damage, has longer skill duration, does not rely on burst, does better against multiple opponents, has bigger skill range, and does not rely on elemental reactions (not consistently procing Fischl's A4 is a huge damage loss).
There are specific cases though which makes Fischl better than Yae Miko as an off-field damage dealer. In particular, Fischl (especially at c6) is a better teammate for Clorinde or Sethos in aggravate teams, Fischl is better than Yae in taser teams (especially those that include swirl damage dealers such as sucrose or kazuha), and Fischl has more particle generation. Note though that Yae Miko still performs well in these areas! Fischl is just a bit better.
Best artifacts
Yae Miko's best weapons
Best teammates for Yae Miko
Yae Miko's team compositions
Yae Miko Techs
Yae Miko's charge attack does not have ICD. This is important especially in aggravate teams with Fischl. When on field, the player should charge attack with Yae Miko, and because her charge attacks does not have ICD, so long as the opponents are affected by quicken, Yae will always aggravate. Yae can be a Fischl driver this way. The problem is Yae's charge attack animation takes painfully long to end. Yae Miko can quickly dish out charge attacks by performing jump cancels to skip Yae's long animations.