DIAGNOSIS You need to worry about it, and you absolutely cannot keep using it. That plug is a critical safety device—it's designed to melt and blow out if pressure gets dangerously high, preventing an explosion. If it's already deformed, its integrity is compromised.
FIX Replace it immediately. For the Presto 01755, you need a new Presto Safety Overpressure Plug. Turn the lid over, push the old, deformed plug out of its hole from the inside. Clean the hole. Take the new plug, wet it with a little water (this helps it seat), and press it firmly into the hole from the inside of the lid until the flange is flush.
IF THAT DIDN'T WORK There's no alternative; a damaged plug must be replaced. If the plug keeps deforming under normal use, your heat source is way too high for pressure canning, or the pressure control system (gauge and regulator) is failing to vent steam. Address that root cause after replacing the plug.
HEADS UP This is not a optional part. You must check this plug before every use. If it's cracked, soft, or looks different from a fresh one, replace it. They're cheap insurance. Never, ever attempt to seal or block this hole with anything else.