How to maximise star utility

You don't remove mash to get her more crit damage, you remove those with the same or higher star absorption to make it such that a servant would have a higher crit chance, not crit damage, but yes it works like that the crit stars would go to Chloe then mash, then Tama cat, and Chloe would have a higher chance in critting, but sometimes it would go to mash by my experience albeit rarely. Though I think it's better to use star absorption CE than to destroy your team comp like this and more than that Chloe isn't a crit servant, she's an NP spammer with an evade having a crit strength modifier as an extra cherry on top.

It is actually very complicated. I will try my best to make it shorter, if still not good since English is not my native langues, then I feel sorry for that.

The star absorption number means how ability of your servant will adsorb the star. Like if you have 20 stars, almost 0 star for berserker if a rider is presented, since the basic star adsorption rate is ~210 vs 10.

However there is "distribution of 90 star adsorption rate for 3 random cards" (50-20-20-0-0), no matter what type of 5 card and what classes these servants are.

I will make a example based on CN Forum
Jatler's (29) teamed with dual Merlin (49), each Merlin has 2 cards where Jalter only 1 card.
Overall star adsorption = 29+49×4+90=315
Jalter's adsorption could be either 50+29/20+29 or 0+29, which means,
The possibility of each star to Jalter's 1 card:
(29+50)/315=25.08%, with 20% chance
(29+20)/315=15.56%, with 40% chance
29/315=9.21%, with 40% chance
It is quite random, Jalter's best luck could be 1 star of 4 overall, which means if you have 20 stars, she may posses 5. But if in the worse case, she she can only have 2 of 20.

So what happened if she activated her 1st skill as 800% added?
29×(1+800%) = 261
Over card all star adsorption = 261+49×4+90=547
Jater's star adsorbtion:
(261+50)/547=56.86% (best luck, 20% chance)
(261+20)/547=51.37%
261/547=47.71%
Jalter will receive 1 star of her 1 card, per 1.76-2.10 stars
Therefore if you have 20 stars, theologically 9.5-11.4 to Jalter's this card. So critical damage is almost definitely.
Off course, later on there will be more complicated, like 9.5 to 9 or 10? and what happened for that 1.4 star?

So as a player we dont need consider that so much. Math is a bad civilization. You just need to realize something like:

Why supportive rider and archer are not frequently presented as normal team? Only Georgies for taunt reason and Martha for debuff removal reason.

Why 2030 Nightingale is also amazing as support for Buster-oriented DPS?

Why the same skill, Saberlot can steal almost all the star, but Raikou cannot do that far like steal stars from riders? (6100 vs 549) But Jalter's 1st skill is probably the worst for star adsorption?

Why normally a critical team is DPS+dual support configuration, not dual DPS in same class? However it is possible to have riders with assassin/lancers as support instead of casters?

Why Sparta is my favorite 2 star ....Nooooo. Why Leonidas is also a amazing teammate for critical berserkers like Raikou, rather than Georgies with 3 round taunt?

etc.

And Kuro + Mashu is not recommended, beside if you have Dual 2030 and the third servant is caster or berserkers.

It works like any other stuff in this game, RNG.
Having a higher star absorption or star weight means the servant has a higher CHANCE of hogging more stars. And I say chance because it is in fact a chance. I've already experienced seeing my Tamamo grab pretty much most of the stars away from my Euryale, which in this case, has a 156 star weight.
And no, you don't need to remove Mash. The number of stars doesn't affect crit damage. It's only for crit chance. If you don't want supports getting a share in the crit stars, find a way to get more crit stars instead.