I honestly don't think that Min Min is the reason that Fighter's Pass 2 may be underperforming. By September 2018, ARMS had sold 2.1 million copies. By April 2019, Persona 5 had shipped 2.7 million copies. While there is a difference between copies sold and shipped, what it does show is that there isn't a huge difference in recognizability between an ARMS and Persona character. Joker may have had a surprise factor and a larger, hardcore fanbase as well, but I seriously doubt those things would make a significant difference in sales.
Timing seems to be the greater issue here. When Fighter's Pass 1 was released, Ultimate was at its peak of cultural relevancy. The game had just came out, everyone and their mom was talking about it, and it was basically the centre of the gaming world. It wouldn't surprise me if many people went out and bought the pass just because they enjoyed the base game so much and wanted more. By comparison, Fighter's Pass 2 came out a little after Ultimate's first year anniversary. It's just an unfortunate fact that the general public's eye moves fast and loose, with many only playing a game for a month before something shiny and new comes along. Sure, Ultimate was still doing incredibly well and had developed a strong core fanbase, but it wouldn't surprise me if people just naturally lost interest as time went on.
There's also the timing of announcements that is likely effecting sales. Let's look at the announcement and release chain of the first wave of DLC content:
- December 7, 2018: Ultimate is released, Joker is revealed.
- January 29th, 2019: Piranha Plant is released, 4-player spirit mode is added.
- April 17th, 2019: Joker is released, stage builder is added.
- June 11th, 2019: Hero and Banjo are revealed.
- July 30th, 2019: Hero is released, online tournaments are added.
- September 4th, 2019: Banjo is released, Terry is revealed, and home-run contest is added
- November 6th, 2019: Terry is released.
- January 16th, 2020: Byleth is revealed, being released 12 days later. Fighter's Pass 2 is revealed.
That's a very steady stream of major content. The wait may've seemed unbearable in the moment, but we were, on average, getting major updates every 2 months. These bimonthly updates had people keeping their eyes on the game and actively discussing it, causing a stronger word-of-mouth about Smash's post-launch support.
Now compare to the second wave of DLC content:
- March 26th, 2020: Fighter #6 is revealed to be from ARMS
- June 22nd, 2020: Min Min is revealed, being released a week later. Rematch is added with Min Min's update
- August 4th, 2020: Small Battlefield is released.
By comparison, we're now waiting an average of 3 months per character update. An extra month may not seem like a lot in retrospect, but longer waits between updates means less talking about Smash, which in turn means less spread amongst public, which in turn means less sales. It also doesn't help that it effectively two updates to reach Min Min, making content droughts often feel longer than they actually were. Smaller updates like Small Battlefield do help to a certain extent, but it just doesn't drum up as much discussion as full characters do.