All classical scholars agree that Sappho was a great lyric poet. The evidence for this is clear.
First, the Alexandrians (who kept classical learning alive) included her in their list of lyric poets. This testimony alone should be enough to say that she was a lyric poet. Moreover, we should keep in mind that the Alexandrians know better than we do. They were close to Sappho.
Second, we have testimony from other ancient authors. They all agree that Sappho was a great lyric poet. For example, one of Catullus's poems was a remake of one of Sappho's poems. Catullus 51 was based on Sapppho's poem. Here is a short except:
Ille mi par esse deo videtur, ille, si fas est, superare divos, qui sedens adversus identidem te spectat et audit...
Finally, even though we have lost most of Sappho's poetry, we know that Alexandrians collected the poems of Sappho. We have the outlines of nine books of Sappho's poetry, based on meter.