NGrams does not show a significant preference for "More important," if we search for a text string more specific to OP's context...
Having said that, what the NGram does suggest is that the "ungrammatical" form has been rising in popularity over recent decades. But if OP is concerned to learn and use "strictly valid" grammar, he should use the adverbial form more importantly.
That's because the presence or absence of the word more makes no difference to the formal grammar. You do something (lack merit, in this case) adverbially. Nothing changes if you do it more adverbially.
If you consider the situation without the word more...
Appellant's argument is premature.
**Important, it lacks merit.*
...all native speakers will find this unacceptable (as indicated by the asterisk). But even though in principle the word more makes no difference to the grammaticality, informal (and/or uneducated) speakers/writers do in fact often use OP's version.
One could say there's an implied/elided "What is more" before important (in practice people often would include either that or an equivalent construction). But OP specifically asks what form he should use; it's not strictly necessary to explain why even native speakers often disregard logic/grammar in this context.