The term "casual" is common amongst video gaming communities.
Often it is given a negative connotation, without much thought into how it is being used.
This is sad, as it is a useful way to deliniate what a given player wants from the game and community they are engaged in.
Now, can and is it used as an insult? Absolutely.
So is "noob" or "elitist" or any number of ambiguous terms applied to a player either appropriately or inappropriately with the intention to offend.
Much like "noob" though it is not necessarily an insult, even when said insultingly. It could be the straight up truth.
If you are unfamiliar with the game mechanics, or the abilities of the character you are using, it is entirely appropriate even if it's hurtful. The amount of effort you expend while playing is irrelevant, only your level of competence matters.
With "casual" the delineation is even MORE helpful, as it is a declaration of how you intend to play.
Believe it or not, most people are not literal monsters that want to upset everyone they come across.
Much of the venom spewed in online play is vented frustration, not malice.
If everyone is aware you are a new player, you will often find more help than you find insults.
If everyone is aware that you play casually, you will not be expected to contribute as much as a hardcore player (the downside being you will not be included when such dedication is deemed necessary)
Now, if a player joins a public game, uses a random party tool or solo queue and immediately expects everyone to be intimate with the game, their role and the tactics that are somehow never divulged by this individual; that player is not "hardcore." They are not "elite." They are an idiot, and the every toxic element that causes much of the frustration you will see. (if they ping the map repeatedly when there is no possible way to do anything about it, they are in this category. They want to show you how clever they are by predicting things that have already happened)
If on the other hand a player tries to organise the game through chat, pings the map to give direction or even invites everyone to use voice comms, that player is your BFF as a casual. He might respond poorly if ignored, he might get into a scrap with the idiot of the previous paragraph (making them have to distinguish sometimes) but if you say you're new or aren't sure what to do, you'll most often get a couple of friendly hints and a pep talk.
Don't run from the "casual" or "noobie" labels. They are your shield from an awful lot of frustration, both your own and those that often seem out to get you.
Embrace your attitude to gaming, the fun you bring will often be infectious.
Most gamers, in my experience, delight in being more knowledgeable and being able to show people the ropes.