No, the furnace's vent pipe should never be sealed with mastic and wrapped with insulation. If there was a damage, corrosion on the vent pipe, it should be replaced with a new one / not patched with any type of material.
If it was a single wall vent pipe
installed in an unconditioned space (i.e., attic, crawlspace, garage, etc.) it should be replaced with a double wall vent pipe (B-vent).
If this is an old vent pipe you're referring to, it might be patched and insulated with asbestos based material so make sure you determine it before any attempt of dismantling it and disposal. Send me a picture if possible.