How can I mask the flavor of frozen vegetables in soup?
What you are describing is often worst when the vegetables are thrown together without care as to what vegetables will do well stewed for a while, and which vegetables only need to be heated through and will suffer if they are cooked longer.

A great example of that is in the case of typical "frozen mixed vegetables". Carrots are never nicely tender in mixtures like that. Lima beans are downright chalky and peas are total mush. If you want to use frozen vegetables, avoid mixtures.

Pick the vegetables that you actually enjoy, and add them to your soup at a time appropriately distant from the time that you actually want to serve the soup. So bring lima beans to a hard simmer before you add corn, bring that to a hard simmer before you add frozen potatoes, add frozen carrots, bring that to a hard simmer before you finally add peas just before you serve - your soup will be better than emptying a bag of mixed vegetables into broth.