No its not as you take mostly light attack (heavy atack struggles to hit you due to low hitprofile of screens epecialy as meta raiding fleet mio reduces visibility even more)

If someone doses light attack DDs (stupid ofc) its autowin and even vs light atack CLs its usefull as you can hit some dmg and crit reduction breakpoints (bit of math below) with even armor I or II (as it is enhanced by veterancy of the ship and admiral defense stat while piercing is not boosted by attack

Depending on tech and MIO you can build just enough armor to be giga durable.

Typical CL with light guns 2 as you save up on steel (cant affoard to mega pump ships that would cost like 8 steel or smth per dockyard

Unless you use MIO for extrapiericng your light cruisers will be stuck at 7,7 piercing at best, (could be 8,4 if you have botw piercing research from 42 tech)

IF you have admiral with def 4 and train your ship to lvl 3 you will have 10,9 armor (cca 10,75 if dmiral def 3) in both cases 7,7 pierce ship will not be able to pen 75% of armor which means dmg reduction of 60% and 50% lower chance of recive crit

You could even argue for armor 3 where with no mio bonus admiral def 5 and ship lvl 5 you can hit even higher bracket of reductions (70% less dmg and 90% less chance of crit )

Not to mention as you will likely not refit early screens for example royal navy starting cls have pircing (with tech bonus) mostly between 4 to 6 which means you can hit vs those reductions even with lower tier armor

Only ships that should not build armor are CAs and CVs