When using HMAC, does key encoding matter?
The theoretical problem with using text encoded keys, is that it doesn't necessarily conform with how the keys are assumed to be formatted in security proofs such as this one. If you are using HMAC with an underlying hash function that is still believed to meet the requirements of a cryptographically secure hash function, such as the SHA-2 family of hashes, this shouldn't be a problem. The collision resistance assumption of those hashes should be enough to make your alternative scheme no less secure than conventional HMAC.

However, technically, using keys with special formatting might be a problem e.g. if you are using HMAC-h with a broken underlying hash function h, such as SHA-1 or MD5. Both of these two algorithms are based on simple operations such as bit-wise logical operators, addition modulo $2^{32}$ and shifts and rotates. Considering that both hexadecimal encoding and base64 encoding results in octet strings with the high bit of each octet consistently zero, it wouldn't be completely unrealistic that HMAC-h with $K_{hex}$ or $K_{base64}$ might get broken before (or after) HMAC-h with $K$ selected uniformly from $\{0,1\}^k$.