I'm seeing a fair bit of misinformation going around regarding #XiaoZhan's popularity, so instead of using opinions, I'm going to be using data and screenshots and hope that hard facts will clarify everything.
Pre-Untamed, his nickname in c-ent was "Top of the Unknowns"

Disclaimer: The purpose of this is to clarify some misinformation. It is not my intention to downplay the breakthrough The Untamed has given Xiao Zhan, or to discredit the drama fans. This will mainly focus on the data from his acting career until end of 2019.

Pre-Debut: 8 Mar 2016 (His debut date was 28 Sep 2016)
Nominated for "Most Popular New Idol" in "TOP Chinese Music Awards"
This award (2001 to 2017) was organized by Beijing Enlight Media. This voting ended with him in #2.

Superstar Academy: 29 Sep 2016
On 4 Nov 2016, the drama Weibo concludes their results in Tencent's 2017 V Vision Event. The drama was #1 in the New Star Web Drama Chart. Xiao Zhan was #9 in the "Watch Him Only" app function. Also, Superstar Academy hits 800m views on 7 Nov 2016.

More on Superstar Academy, some charts that I got from archives. On 2 Oct 2016, the drama was #2 on Duguo's Web Drama Viewership, #7 on Heat Rating and Xiao Zhan was #8 on Web Drama Artists Heat Rating.
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Oh! My Emperor: 24 Apr 2018
This drama is his first mini breakthrough. Datawin on 9 May 2018 places this web drama on #10 on the chart, with Xiao Zhan contributing 63% of the heat. On 19 May 2018, he joined Happy Camp as a guest star.

In addition, in terms of Vlinkage Chart, Xiao Zhan went onto this chart for 21 times, peaking at 6th position for his role as Beitang Moran on 11 Jun 2018.

Battle Through the Heavens: 3 Sep 2018
Xiao Zhan had a very minor role in this TV drama, but was still contributed 21% of the heat on this Datawin Chart on 17 Sep 2018.

Doki Chart: 18 Dec 2018
Xiao Zhan won the 2018 Tencent Doki Popularity King award of the year.

2018 ended with him rated #22 on Datawin's 2018 TV Series Rising Energy Chart with the caption - "Pending Fame"

I-Magazine: 16 Apr 2019
I-Magazine announced their "Fashion Face Award Year 2018", where Xiao Zhan came as #2 for Asian Male.

The Untamed: 27 Jun 2019
Some pre-airing data first. On 2 Jun 2019, The Untamed breaks 1.5 on Datawin's Pending Drama Heat Rating and the main source of the heat comes from Xiao Zhan. Peak heat rating occurred on 25 Jul at 4.5, with him contributing 53% of the heat.

Jade Dynasty: 13 Sep 2019
The movie was top of China's Box Office for their Mid Autumn Season, Xiao Zhan was scored 9.2 in Datawin's Artist Rising Power Chart.

Joy of Life: 26 Nov 2019
Xiao Zhan's role in this drama is very, very minor, but the effect could still be seen in Datawin's Dec 2019 summary, with him topping the Artist Internet Influence Chart with the caption "Yan Bingyun" with a score of 8.9.

2019 ended with The Untamed rated as the #2 on Datawin's 2019 Drama Prosperity Index with a score of 2.66. In the 2019 artist charts, Xiao Zhan was placed #4 for Artist Internet Influence (8.4), #11 for Artist Drama Commercial Value (8.0) and #1 for Artist Rising Power (9.8).

Last bit, Weibo followers increment chart... after putting it in a chart I'm also amazed that the biggest surge of followers actually came in Sep 2019, while Jade Dynasty was airing.