How bad is using a 200 amp breaker box on a mobile home that only has 100 amp underground wire?
Service lateral

The service lateral (underground version of the service drop) before the meter is the power company's responsibility.   They have a smart meter on it.  They know what is installed in the ground (it could be #4Al for all we know) and they know when you draw on it hard enough to cause thermal problems or voltage drop.  So go ahead and install your Bitcoin miners and horticulture lights if you're in a state that's into that... The power company will let you know if you need a service lateral upgrade.  

Service laterals are not subject to NEC.  The power company has a different codebook.  

Your responsiblity is to have overcurrent protection (breaker) that is appropriate to the power the power company has promised to provide you (provisioned you) i.e 100A.  The service lateral may be wholly inadequate to the task if you really drew all that; but it's their right to undersize the cable on the bet that you won't, and their job to bird-dog that. 

A downline (sub) panel can always have a bigger breaker

Since you have a 100A main breaker at the meter, it's perfectly fine to use a larger 200A panel downstream of it.  There is nothing that requires the subpanel's "main breaker" agree with the breaker that is actually feeding it.  


if the subpanel's "main" is bigger than the supply breaker, it is merely a shutoff switch.  Given how panels are priced, it is often the cheapest way to get a shutoff switch if one is required (e.g. In an outbuilding). 
If the subpanel's "main" is smaller than the supply breaker, then each has different jobs.  The supply breaker protects the wire/Cable to the subpanel.  The subpanel "main" protects the subpanel itself.  


In this case, since the "sub" panel is in fact the domicile's main panel, using a 200-225A panel is the only sane choice.  100A panels are quite small panels, typically not much larger than 20-space.  You need as a "desperation minimum" 30 spaces in a main panel, and 40 spaces or more is a better choice. That is especially so in an all-electric home, where otherwise-gas appliances are gobbling up breaker spaces 2-4 at a time. 

The traditional desperation move when you run out of spaces is "double-stuff" breakers, but that is becoming less viable as more and more circuits require AFCI or GFCI.  Those breakers are not available in double-stuff.  Given the price difference you are far better off just buying a 42-space or two 30’s, rather than painting yourself into a corner and having to spend much more to get out of it.