Are there certain muscle groups cyclists need to pay more attention to because they are not used in the act of pedaling a bike? I'm starting to develop some hip problems and I wonder if I may be over strengthening some muscles over others.

I focus on the back and upper body in general when I go to the gym. A lot of cyclists have all of their muscle mass in their chest and legs.

Hamstrings and the upper body are nice. However, I stick to an all around program.
I grew sick of having decent legs while everything else was crap.

Probably one of the most important areas to work as a cyclist is your core. I do three sets of three abdominal exercises every time I lift at the gym.

When it comes to strength training targeting the whole body is always a good approach.

Nope. Hate to say it, but strength training is not a great use of time to get good a cycling.

That hour you spent working on your 'core' would be far better (for cycling, specifically) spent doing hard intervals or hill climbs on the bike. Just look at all the top cyclists, and see how slender they are - twig arms, and surprisingly small quads.

Short-track sprinters , however, require big muscles. That's a different forum, though.

I hate that pro-cyclist look myself, and lift weights. But I'm under no illusion taht it's helping my riding.

It doesn't seem like OP is asking for strength training to improve his cycling, but to make sure he won't hurt himself through uneven muscle developments. I personally had such a situation happen to me - I was a competitive swimmer, which strengthens the muscles in the shoulder rotator cuff that rotate inwards (for the pulling through the water motion) but did nothing for the muscles that controlled outwards movement (think a tennis player's backhand). Over time, this caused caused impingement and tendonitus (eventually a tear) in my shoulder (which is no fun).

As for OP's question, I'm too much of a cycling noob to answer.

core work helps the most.

also use a foam roller on the hip flexor and legs

I agree that for general fitness, core strength is a good thing, as it contributes to 'general' function in the world, be it shoveling snow, lifting groceries, or playing pickup basketball. However, if you truly want to get good at cycling, you have to ride - a lot - to the point of exclusion of other sports. There is a reason why triathletes can never match the ability of a TdF cyclist in cycling, despite sometimes logging similar miles/hours and having an obviously far superior core due to the swimming/running aspect. Similarly, a gymnast who has incredible core strength, with no endurance sport background, would get crushed in a local race.

For endurance sports, to get good as fast as possible, and/or to reach your best potential, you have to do the sport nearly to the exclusion of other activities. Other activities won't hurt, but they will limit your growth in the sport at some level. Fortunately, for most nonracing cyclists on this forum, that's perfectly ok. But even for a locally competitive cyclist or endurance athlete, it generally requires exclusive-type training. If I spent 2 hrs a week lifting weights, that would mean the difference between winning my age group in a run contest versus placing out of the top 3. Same in cycling - if I spent 2 hrs a week extra training hard on the bike as opposed to lifting weights (and thus a 4 hr cycling differential), it could make the difference between hanging with the front pack versus getting dropped.

Yoga is helpful as well. I have had a few issues that I ironed out with it.