The planner takes the time needed into account. It should normally be an hour or more. Check a Paris metro map (e.g. here) to see how to get around. From Paris Montparnasse to Paris Nord, you can take metro 4 from Montparnasse Bienvenüe in northern direction. Metro tickets are normally available at the TGV bar.
It is a VERY long walk to metro there-you can go other way for shorter wlak-and change to RER.
SNCF always uses at least 1 hr for the transits.
Do NOT buy tix from TGV-bar-they often are out of date. Just buy 1 (1,90) from the very few remaining paper ticket dispensers in metro-hall-keep till end, in Nord you often have to also pass gate with it to go out. RATP intends to scrap these old style tickets since a long time and this may happen very soon now.
IF you have not yet booked anything-find a direct TGV Brd-AMIENS or even LILLe-and change there=same station (or in LIlle may be not, but short walk)
Do NOT buy tix from TGV-bar-they often are out of date.
Never heard any complaints about that. At most that they were sold out.