I’m a home mechanic but not a professional.
Depends on what you mean by “not good”. If your car is using 1 quart of oil every thousand miles or it sounds like there’s a woodpecker under the hood, that’s “not good”.
There are several possible reasons for your problems, none of which can be diagnosed without having the car here. Modern cars are sealed systems from the air cleaner to the tailpipe. No air that goes into the intake is
There may be a vacuum leak. One of the rubber hoses on the engine may be cracked or loose. There is a sensor just beyond the air cleaner that measures how much air is entering the engine. The car’s computer calculates the correct ratio of air to fuel. If air leaks in beyond that sensor the calculations are wrong.
There is an Idle Control Valve on the side of the intake manifold. The simple explanation of this device is that it calculates how much air to feed the engine based on whether the engine is cold and how much load is on it. If you have the A/C on, or the car is in Drive, there is more load on the engine. As part of the sealed system it bleeds off air depending on the needs of the moment. Since it’s Summer, I assume you have the A/C on. This valve can get gummed up with fuel residue and the valve isn’t able to function correctly. It has to be taken off and sprayed with fuel system cleaner.
These are two of the more frequent causes. Your engine isn’t in dire shape and the cost to repair depends on how quickly the mechanic can find the cause.
If you just had the 30,000 mile check done by the dealer, a check of the vacuum hoses is usually listed but rarely done. They last beyond 75,000 miles in most conditions, (Viton replacements will last the life of your car) and unless they heard the hiss of a loose one they wouldn’t spend more than a minute on them. They may have also said they did a fuel system cleaning (usually poured a can of cleaner into the gas tank) but that would not have done much for the Idle Valve. They’ll deny any negligence and charge you to fix it. Bring it to an independent garage (if there are any left near you).
As an aside, all brand name gasolines have detergents in them. This mainly benefits the combustion chamber. Fuel deposits will still accumulate in the intake system and around the valves but it is much less than what used to occur. You don’t need Techron or any additive for the gas tank. They will not do anything.
There are other additives that can be added into the intake system that reportedly clean deposits. I have no proof for or against those claims but I personally feel better when I feed Seafoam through my intake every 30,000 miles.