Is a 1984-S quarter with a bubble on the obverse and is thicker at the top a mint error?

It may be a Mint error only in that the coin is an uneven thickness, in which case it would be a planchet error. If the coin is not damaged otherwise (excluding the bubble), though, then it's probably not a provable Mint error. The bubble is probably a lamination problem, which can also be a planchet error. This occurs when the cupro-nickel layer (the "silver" colored layer) separates from the copper middle.