Some stuff about the Mark I've observed.
It isn't mere decoration.
It isn't there for show, it actually grants power -- power of darkness.
The power exemplified is dependent on how much the user taps into the darkness. A non-violent (tame) looking mark is the most basic form of this POD. The 10c all have this stage activated 24/7 like Itachi with his Sharingan (maybe by actively leaving it like this 24/7, they incrementally gain great mastery over the POD in general, but who knows).
The mark's design changes or spreads depending on how much POD the user is tapping in to. For example; Derriere's tame-looking mark was starting to protrude in a violent fashion when she confronted Nerobasta; meaning she was powering up.
When Nerobasta used
ARK on Derriere, Derrierie was screaming in pain (she was hurting badly). Monspiet was very concerned as it looks like she was even decomposing
It wasn't until she activated an even great portion of her POD, powering her up even more, did she become able to break free of ARK.
After having her revenge on Nerobasta, Derriere powered down & her mark returned to stage one (tame).
Whether or not she has a fully demonised form like Meliodas remains to be seen, but we have sufficient evidence to affirm that her mark does increase her PL like Meliodas' own mark increases his.
As for why Estarossa or any of the Commandment didn't activate a powered up version of their POD form during their fights is really is anyone's guess. I guess not all of the 10c could control the POD as proficiently as Derrierie -- perhaps they'd also lose their mind like Meliodas' did at times if they tap into too much POD. I also guess that that it's due to superiority complex. The 10c consist of many arrogant individuals that believe everyone else not of Goddess blood is automatically inferior to them. Their lackadaisical attitude towards anyone not of Goddess blood is disadvantageous for them during fights as it lets others get the drop on them.
I think you're underestimating the Supreme Demon considerably here.
Peak Meliodas is still a complete mystery. All we know is that he was the Strongest Demon.
I think you should also take into consideration that not all Demons have the same lvl of power in base, thus the 10c wouldn't necessarily be comparable with their POD form activated. Demonised (tame) Estarossa only has comparable PL to current BASE Meliodas, who hasn't even reach Supreme Demon status yet.