As far as I know, the base dps of weapons and clothes is dependant on the level of V when its first spawned.

The dmg of weapons picked up at lower levels drops off hard when taking on the higher threat enemies in the game.

The brute force solution is to simply avoid grabbing any iconic weapons until you power level to lvl 50

After exploring the options available in the game I've found that there are 3 methods getting a weapons levels up to the current level of the V. All of them require some investment into tech.

1. Rarity upgrading.
With a few acceptations, most iconic weapons are found or crafted in a rarity below legendary. When a weapons is upgraded from one rarity to another, the stats of the weapon is adjusted to the current player level. May be a good idea to hold off on upgrading to legendary until you're level 50 if you want to save on materials.

2. Regular upgrading.
The simplest but by far the most resource intensive method. Found in the crafting menu near the top, this method upgrades and item by one level at a time. each upgrade with cost more than the last so it can quickly become too expensive to be worth it unless you are absolutely in love with the weapon. note that items acquired by a level 50 V can be upgraded once and then no more.

3. Direct Crafting (craftable iconics only)
The iconic items that are purely craftable are set to the level of the V that crafts them. Most of these weapons specs are found on the big boss of organised crime alerts. simply scrapping them and remaking them lets you get the same iconic at your current level. may need to remake a few time if you want a god rolled weapon but its still wildly cheaper than normal upgrading.

Those are all the legitimate methods of changing Iconic weapon levels I know of. Think there may be some janky way of fixing these levels by using V's stash but I don't know if its reliable and it may get patched anyway