You would probably get a better answer on a forum specialized in encryption, but here’s a couple of thoughts:
The “strength” can be considered as one of two things:
How easy or hard it is mathematically to crack (guess) the password, given how the password is constructed.
How easy or hard it is for a person or program to crack (guess) the password using knowledge of how people make up easy-to-remember passwords.
#1 would depend on the cryptographic algorithm, and I know far too little to comment on how sequential letters would impact the cryptographic strength of the password.
#2 is a bit easier: When people are asked or forced to use a mix of letters and numbers, many choose to simply put consecutive digits at the end of the password, e.g. xYz123. Knowing this, it’s almost as easy to crack the passwords xYz1, xYz12, xYz123, xYz1234 and so on, as it is to crack the three-letter password xYz - which is trivial to any brute-force password-cracking program.