Sauce tastes overly acidic or sharp

Cosmetic

Usually underripe or naturally sharp tomatoes used without the balancing effect of cooking, salt, or fat — the counterpart to sauce being too sweet, in the opposite direction.

Ask yourself

How ripe were the tomatoes, or was this a lower-quality or unripe canned product?
Was the sauce cooked down at all, or used straight from the can/tin?
Was any sugar, salt, or fat used to balance the acidity?

Likely causes

#1
Underripe or naturally high-acid tomatoes with nothing added to balance them
Strongly supported
Tomato acidity varies significantly by ripeness and variety; a sharp batch used without adjustment tastes noticeably more acidic than a well-balanced one.
#2
Sauce used raw or barely cooked, without the mellowing effect a longer cook provides
Generally accepted
Cooking a sauce down for longer rounds out sharp acidity as some volatile acids cook off and flavours concentrate and meld.
#3
No counterbalancing salt, fat, or small amount of sugar included
Generally accepted
A pinch of salt or a small amount of fat (olive oil) can round out perceived sharpness without needing to oversweeten.

Rescue it now

Not fixable once baked; a drizzle of good olive oil over the finished slice can help offset sharpness at the table

Prevent it next time

Taste the tomatoes before committing to a sauce style, cook sauces down further if they taste sharp, and adjust with a pinch of salt or fat before reaching for sugar

Cooked tomato sauce

Go deeper

Guides that treat this as an outcome to improve, an environment to work with, or a flavour to understand, rather than only a problem to fix.

How to get sauce to behave better Raw sauce vs cooked sauce: how they taste different