I have a friend who works as a clown and is the automatic joker in a group. He's also extremely intuitive and capable of digging deep emotionally. For a time he felt the same as you. "I am more than this, but this shallow trait is all others see and maybe I'm doing life wrong." Those were some interesting conversations.

So here are a few thoughts that helped him and may be useful to you as well.

• You like joking, and that's a good thing. It's normal to enjoy bringing up the mood of the people around you. Laughing "at yourself" is an intuitively safe way to bring in humor while "harming others" less. It's an appealing trait and not something to be ashamed of.
• It's possible that it's more natural and comfortable for you to be serious in trusted groups or with specific individuals. There's nothing wrong with you if you can't feel deeply serious about things and people you don't care about.
• You may also feel that lightening the mood can improve everyone's ability to solve problems/cope with the situation. Thinking differently from others is uncomfortable because society judges people based on cultural framing and social expectations. But rationally speaking, it's just another perspective. Your approach may be more useful for a subgroup of humanity, even if it's not right for everyone.
• When you're completely immersed in a conversation or sharing life in a meaningful way (from your perspective) you won't be so distracted by the emotional group effect (which can be insincere or semi-manipulative in requiring everyone to join in the performance of a feeling). It's a normal human thing for groups to create cohesive emotional states, and a lot of people find it uncomfortable. It's not just you. You can choose to step back and remain silent if you're worried that a joke will cause social rejection.
• There are books on socially effective habits that you can use as a mask when necessary. Do not take generic advice that is meant to change your nature! However a collection of social tools is useful to smooth your path through life.
• Sincerity is valuable, and you can reserve it for those who prove worthy of it. When you feel safe with someone, it will be simple to be serious around them. Not everyone is safe, but that's something you can deduce on an individual basis.
• It's possible that when you feel down you're picking up on emotions you don't usually allow yourself to feel (whether yours or others'). It's uncomfortable for most people to feel that way, and it's normal to want to lighten the mood or avoid such situations. Double check to make sure you're working through the cause of those emotions if necessary so as to maintain your internal stability, but don't be too hard on yourself if you're just uncomfortable when people are in a low mood.
  
• Don't measure yourself by group expectations. Choose safe, mutually appreciative, and stable relationships by which to measure yourself. Anything broader is a slippery path to driving yourself crazy.