Depends a lot on the party
CR doesn't really represent the difficulty of a monster (a CR2 Intellect Devourer can insta-kill a lot of high level character on a bad roll) but it help to apprehend how difficult a fight might be. Using Kobold Fight Club, lets compare some difficulty thresholds :
A Mummy Lord alone against 6 players is 6'500 XP (13'000 x0.5 for the party size) : a Hard (almost Deadly) encounter for the weakest group and an Easy one for the strongest and just a Medium one for the average.
The fight difficulty doesn't take some data into account, like the damage types available to the party : if everybody have fire damages, the poor mummy won't last long while it might be difficult to hurt it if the melee damage-dealers don't have magic weapons (at lv5, it's a possibility).
Action economy tend to advantage groups against a solo target. Legendary Actions partially counter this advantage against a "standard" group of 4 but I don't know if it will be enough against bigger groups like yours.
So, is this a good boss fight ?
Honestly, I don't think so.
The Mummy Lord feel like a heavy hitter but not much a tank (even with its high AC), which isn't what I think a boss fight should be :
If the party is low level, the Mummy Lord can "One Hit Knock Out" (even kill the frailer characters) with its main attack (4x a round with Legendary action). +9 to hit, (3d6 + 6) + 6d6 = [min 15, max 60] with a chance to not being able to be heal until uncursed is kinda high at lv10.
If the party know that basic Mummy are vulnerable to Fire, they might try it on the Mummy Lord. It only have 97 HP and +0 dex saving throw : an average Fireball do 28 fire damages, so 2 Fireball might transform the Mummy Lord in ashes (56 per fireball with the vulnerability).
If homebrewing doesn't bother you, do it yourself
I feel like it's a bit off-topic but I wouldn't recommend the majority of the RAW monsters as bosses. There is a lot of homebrew system allowing to create "boss monster" like the AngryGM's or Giffyglyph's Monster Maker. I used AngryGM's to create a multi-phase boss for my party when they were lv3 (a goblin possessed by a ghost) and my players still remember this fight, almost 2 years after.
When I want to create a boss fight, I list few things :
What type of monster the boss is ?
What "role" it has ? (describe the monster main course of action. Does it support some minions, control the battlefield with area of effects or is a melee tank)
What kind of phase it has, if any ? (a bit video-gamey but show the players "there is some progress" + add some variation in a somewhat long fight)
What action can disrupt it and how ? (allow the players to remove some actions or effects of the creature)
Once this set, I use a homebrew system or the Dungeon Master Guide to create the monster with the difficulty you want. I tend to go for Hard encounters for boss fights.
In your case, I would recommend to create the same monster multiple times, changing its stats to be a Hard encounter for different party levels, and choose the most adapted one when the group enter the "boss room" (depending on their resources, you might want to crank up/down which one you take).
Example for a Mummy Lord boss with 2 phases :
Conjure lesser mummies to attack and swarms of beetles to apply debuff in area
Goes berserk and surround itself with a sandstorm doing damages and reducing visibility.