Both are acceptable, as @BillJ said in his comments. And to prove him right, I will just quote American Heritage Dictionary that addresses this problem in a long usage note for "what"(especially point 3):
Clauses with what as either subject or object may themselves be the
subject of a sentence, and sometimes it is difficult to decide whether
the verb of the main clause should be singular or plural.
what in the what-clause is the object of the verb and the complement
of the main clause is singular, the main verb is always singular:
*What they wanted was a home of their own.
When the complement of the main
sentence is plural, the verb is most often plural:
education needs are smaller classes,
though one also encounters sentences such as
What the candidate gave the audience was the same
old empty promises.
When the verb in the what-clause is
singular and the complement in the main clause is plural, one finds
both singular and plural verbs being used. Sentences similar to both
of the following are found in respected writers:
What drives me crazy
is her frequent tantrums.
What bothers him are the discrepancies in
When the complement of the main clause consists of two
or more nouns, the verb of the main clause is generally singular if
the nouns are singular and plural if they are plural:
What pleases the
voters is his honesty and his willingness to take on difficult issues.
On entering the harbor what first meet the eye are luxurious yachts
and colorful villas.
You will never go wrong if you use your verb in the singular in "what is important is". However, we cannot dismiss as incorrect the use of the verb in the plural (i.e. "what is important are" + plural predicate) because of its plural predicative which follows after. We must simply be aware that it is quite uncommon, as you can see in this NGram:
There's no doubt that the subject of these sentences is what's most important, not good customer relations. If the speaker had intended to express the subject as plural, they should have said what are most important instead, in which case the subject should have been followed by are good customer relations:
What are most important are good customer relations.
But the speaker didn't express the subject as overtly plural. So there's no need to make the matrix verb plural just because the predicative complement good customer relations is plural.
Now, some speakers might think that the subject is good customer relations and use the plural verb are as in the second sentence.
What's most important are good customer relations.
But I'd think these speakers are confusing it with a legitimate inversion where the subject is indeed plural good customer relations: