Does "Thanks a Lot" sound too casual for showing gratitude to someone in a higher position

I'm thinking probably... because of "a lot"

Not only does it sound informal, but both Thanks! and Thanks a lot! are frequently used ironically - frequently enough that they should be avoided anytime there's even a slight chance of ambiguity. I would never use "thanks a lot" to my boss or supervisor unless we also happened to be friends.

It would be hard to go wrong with Thank you, Thank you very much, or Many thanks.

Yeah, it will probably sound a bit too casual, if you are addressing someone in a higher position.
Not unless the person who is in a higher position doesn't mind, or you and that person are in close standing. Otherwise, I would say:

Thank you very much!

Depends what you mean by "higher": if it's somebody you'd still be on fairly informal terms with (e.g. first name terms, or non-honorific terms if your native language has such a thing), then "thanks a lot" is fine. If the context demands use of more formal language, or if you're in doubt, then you can't go too wrong with "thank you very much".